Most dwarf boas will be above your price range. Plus, you will need to ultimately get a well insulated tank for heat and especially humidity requirements.
Check out Vin Russo at Cutting Edge Herps to see prices as well as Tommy Carpenter at TC Reptiles. I bought Feliz, my dwarf Venezuelan BC from Tommy and had a great experience. He is happy to talk on the phone and I believe his number is on his website.
Both guys are great breeders.
You also want to be careful with what species and individual you get because temperaments seem to vary more in dwarf boas than columbian BI's.
Let us know what we can do to help.