My headcanon is that Eddy fell for Edd the moment he spontaneously decided to give him 2 quarters instead of 1 (this is even part of my fanfic about how the Eds first met, the one I've developed the most so far actually)
I'm sure he was like "Hello?? Where have you been all my life?? I love you???" xD
Although it's a funny idea, I can see it making sense. I find it amusing to think that Eddy's love for money sparked his unconscious crush on his future best friend lol (it's not exactly the only reason he fell for him, but it was more or less the starting point).
Of course, initially he sees Double D as a good way to make money (both because he's gullible and because of his construction skills), without realizing that deep down he also has feelings for him.