I noticed you were from MA, I would recommend calling Zoo Creatures (it's the shop below N.E.R.D). That's where I got my BRB, and she is excellent. I bought her a few years ago for $250, but I had to have her, and that was by far my most expensive snake purchase. I've seen them for sale at the New England Reptile Expo as well, there were quite a few that were in the $125-$150 range. But I would still recommend calling the reptile place I mentioned, I'm not sure if they are known for BRB's, but I haven't seen a friendlier and nice looking snake than the one I got from them. But call first, they might not have any.
Also, I'm not sure what type of Rainbow Boa you were interested in, I assumed Brazilian, but their are others. You can get Columbian Rainbow Boas for around $60-$75, but they aren't colorful like the Brazilian's (more brown, but still nice looking).
Good luck with your search!!!