If price is an issue I wouldnt even get one. Honestly the smallest and generally most docile are the dwarf locality boas. Like the Hogg Island boa. They stay a couple feet smaller and grow slower if fed the right way. They are priced a little higher but I have seen baby hoggs going for around $100.00 give or take. The lighter and less speckled ones go for a bit more. A regular BCI is definately alot more affordable but dont just get one from a pet store if you are not familier with where the snake came from and what its health is.
Just stay away from the BCC's if size is an issue. I have 2 BCC's and it is not unheard of for a female to reach 10ft. They are also thicker and "taller" than a BCI. Meaning the body is higher from belly to spine and more rectangular shaped.
If you get one just be sure you have everything you need beforehand and be prepared to upgrade the housing at least twice before you settle on an adult boa cage at about 3 to 4 years of age.
Look at this link to see a pretty good list of dwarf boas.
http://www.cuttingedgeherp.com/boaconstrictorimperator/
It seems the Tarahumara Mountain Boa is the smallest. Just read all the details for each of these and you will have alot of info.