Some Affordable BJDs and alternatives from my experience
Time for some rambling from me again. I will only list companies whose dolls I own since I can't give much of a recommendation if I don't have a doll.
Guard Love- Resin company with both realistic and cute anime heads. They have variety of bodies and heads available and you're able to mix and match within the size. They also have a good range of resin colors. I think their anime heads are actually quite varied since they have different mouth and eye shapes. Hertz is a very distinct anime sculpt and they also have a head where you can use decals for the faceup instead of of painting it yourself. Their production times are fairly quick, being around 3-4 months. Your doll also comes in a grey duffel bag that you can use to carry other dolls in. Most dolls are under $300 blank.
Fatemoons - Really pushing it as their dolls are just slightly under $300, but you can get some really nice sculpts. They're popular in their home country of China but not so much on English forums, which is likely due to them being a banned company on DOA. This is due to an account issue and not from recasting or counterfeit issues. They also sell a variety of resin parts such as accessories, wings, limbs, and pets. Their current available sculpts are humanoid, but they do occasionally have a more unusual sculpt such as one of their cat heads, Willow. It's my favorite cat head sculpt and I do have one. Their 1/3 dolls are over the $300 budget and some newer 1/4 dolls are also over that.
Jun Planning Ai - This line has long been retired, but these are tiny 1/12 strung ABS dolls. Single jointed. A complete set has a wig, eyes, shoes, a full outfit set, and some have an additional accessory. Their price varies, but they show up all over the place, usually ranging from $50-100 dollars depending on which doll it is. This is actually my first BJD before I even realized what a BJD was since I found an almost complete Nerine [just missing her teddy] at Goodwill for just $3. So keep your eyes open for them in the wild.
Lots of 1/6 dolls from more expensive resin companies are under the $300 mark although shipping is a different story. A lot of companies also have 1/4 dolls under the $300 mark as well. I'm just naming a few companies that have dolls in this price range. I would say that most resin doll aren't even anywhere near even $500. If, and only if, you are collecting in 1/4 and below scale. I don't have a lot of 1/3 and I can't give you good reference for that.
Most of these are well known, but I'll go over them.
TinyFox - Anime styled plastic dolls. They're not vinyl though, and they're mostly called that due to the lack of proper terms to describe their classification. They are primarily known for selling elaborate full sets, or a doll that comes with faceups, wig, eyes, and clothes. No shoes though. They have a rubbery coating on them that stains super easily and is impossible to fix once stained, but I heard the newer ones don't have this coating. Honestly, I wished they never had this coating to begin with seeing how hideous the stains are on my dolls. Their clothes are well made and elaborate and you can buy just clothes once in a blue moon. These dolls are heavy and their joint system is similar to a ball socket. Think obitsu 11s. Almost all of their dolls are under $300, and they come in either 1/4 or 1/6 sizes. They also sell bodies and heads that you can customize yourself. No boy bodies at the moment though.
Imomodoll - Honest to goodness vinyl dolls structure wise like a Dollfie Dream. I have issues with their quality, but they have a bunch of available options that Dollfie Dreams usually don't have available for everyone. Such as tan skin [it's darker than DD too] and male bodies. They have 1/6, 1/4, and 1/3 sizes, and they also have a small strung doll at the 1/12 size. A complete doll from this company varies depending on size. 1/6 dolls are roughly ~$60, 1/4 ~$90, and 1/3 ~$200. Really depends on your dealer. There are a lot of different heads that you can buy and a good range of bodies with more to be released in the future. Imomodoll also has resin dolls, but they're slightly over $300, so they don't really make the list.
Obitsu/Parabox - A cheaper vinyl doll company selling a whole bunch of different sizes and a whole lot of heads. You can either buy parts individually yourself or get a premade set. Most of these dolls fall under the $300 budget range, although whether this holds true or not depends on your currency's exchange rate against JPY. And while some of their larger dolls like the 80 cm Otohime would meet the budget, the shipping costs are high. You can also select a variety of faceups, clothes, wigs, and eyes for your doll. The only issue with this company is the availability of items, with you needing to wait for several months with no idea when an item will be available. Their obitsu 11 bodies are widely available, which leads me into
Obitsuroids - A whole type of dolls that I would argue is even more customizable than the typical BJD doll. So where to even start with obitsuroids? Most are around the 1/12 scale, although it's been creeping up to 1/8 and 1/6 scales as of late. There are a variety of different materials for this doll, ranging from PVC, vinyl, resin, and even rubbery materials somewhat like silicone. These dolls are called obitsuroids due to the original ones being Nendoroid heads on Obitsu 11 bodies, but it's really used as a catch all term for all dolls with Nendoroid styled faceplates and 1/12 bodies. If you want to get into obitsuroids, keep in mind that you will need something called a headback to complete the head. A faceplate is not a complete doll head. If you get Nendoroid hair, this can complete the head, but if you don't, you will need a headback. You also need a neck card to attach your head to the body and not all companies that make bodies for obitsuroid hybridization [oh i can go on a whole tangent, only mini tangent though] comes with a neckcard. Overall, obitsuroids fall well under the $300 mark, but that really depends on what body, faceplates, hair or headback it has. And it can be difficult for some to source parts, although I'm sure there's resources available for this somewhere. I have a ton of obitsuroids and I will probably make a master post for this one day. The 1/12 blind box BJDs are also structurally based on obitsuroids, albeit modified for proprietary purposes.
Azone Pureneemo and Picconeemo - A line of 1/6 and 1/12 fashion dolls, although the Picconeemo lines and female body Pureneemo lines often have more elaborate joints like an Obitsu of a similar size. They're known for their well made clothes, and the cost of the doll is primarily in the clothes over the actual doll itself. Most of these dolls are under $200 new or secondhand, although some dolls can be over that. However, a lot of their dolls have painted eyes and their hair is rooted in, although I think they have unrooted caps available for some dolls.
Dollce Mini Sweets Doll - Cute little 1/12 dolls. Expensive for their scale and size, but you can find some secondhand ones for cheap. Most shouldn't be more than $150, although I have seen a few like that for really no reason. They use Dollce vinyl heads and obitsu 11 bodies.
If you don't mind second hand dolls and have a currency that's strong against the JPY, Mandarake is an excellent Japanese second hand shop, with some dolls even coming with accessories from their previous owners. There is a risk of running into counterfeit dolls, but Mandarake is reputable and I have bought several dolls from them. You can also find a lot of obscure Japanese brands via Mandarake as well and even parts.
There are way more affordable dolls in the marketplace than I listed here or on other lists, but they aren't accessible, or 'region locked' so to say. But keep that in mind.