Saw someone say that the reason there's no entry grade Zaku is because it would be TOO convinient as an army-builder, and that Bandai makes more money keeping the high grades as the cheapest option. My own theory before reading that was that the pipes were just too difficult to do in a way that doesn't require tools, at least at entry grade price.
Either way as someone who likes giving away entry grades as gifts, I wish there was a mono-eye option to offer to people, and I think that the original GM would be a good contender since it's such a simple design and I presume it's not as popular of an army-builder as the Zaku or more modern variants of itself like the OVA versions. Just look at the high grade. I think you could definately do a version of this that doesn't require tools.