I would say $1,000 as a fairly worst case scenario, but I spent more when I got Yafe (carpet python) who had a full blown RI after fed ex lost him.
However, buying from a reputable breeder means the risk of a sick snake, certainly with parasites, etc. is much lower. Also, with Boas, captive bred is pretty easy to come by and that reduces the parasite risk a lot too.
I don't remember if you decided on a Boa breeder, type, or a particular snake yet, but if you want to PM me, I can point you in some good directions breeder wise.
However, I cannot emphasis enough buying from a good breeder. Also, quarantine is a must with any new reptile.