Definitely check the Rainbow Boa forum to get some more info, but here's my take on things to get you started! Please keep in mind that this info is for Brazilian rainbow boas - I don't know anything about the other subspecies.
"What size do they get when fully grown?"
Females, about 6 feet, males about 5 feet (average). They are a slender snake, much thinner than your ball pythons.
"What is a fair price on a captive bred male/female?"
I've never seen them go for more than $175 at local shows, but I'd say that anything up to $200 would be reasonable. Rare patterns such as stripe or full crescents would be more.
"Do they have good temperament?"
Mine is a total sweetheart and has never attempted to strike, but then again, I've only had her since January and have handled her frequently. She does like to bluff a little when I reach in to take her out of her cage (hissing and curling up), but she's never backed up her bluffs, and I wouldn't expect her to. I'd feel comfortable letting my friends handle her, with careful supervision, of course.
"Are their different "types" of rainbow boas?"
There are many subspecies such as Brazilian, Peruvian, etc., as well as different pattern and color morphs within the Brazilian group - stripe, full crescent, red, and orange varieties.
"Any other useful information?"
Keep the humidity high and the temperature cool. You should be aiming for 70% humidity or more for young BRBs, and temperature in the low 70's to the high 70's or very low 80's (not above 85 degrees).
Rainbow boas act much like ball pythons, in my experience - they are very secretive and inactive during the day and will sometimes come out to cruise at night.
I love my rainbow boa - she was my first snake, and I'm really happy I decided on one. You won't be disappointed!!
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::i believe in joy > http://www.winds.org/nekomi/hope.html
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My Growing Zoo:
1.0 Husband (Byron) ^_^
0.1 black cat (Shade)
0.1 Brazilian Rainbow Boa (Zia)
1.2 Cockatoo cichlids (A. cacatuoides yellow-gold)
1.1 WC Cockatoo cichlids (A. cacatuoides blue Peru)
3.3 Pygmy corydoras (C. pygmaeus)
2.0 Endlers' Livebearers (P. sp. Endlers)