Well, someone with more 'market' experience than me may be along to correct some of my figures, but here's a few points for you...
If you are just looking for a normal patterned BCI (common columbian boa) with no special genetics involves (ie: het for something), then you ought to be able to pick one up for $30-40 depending on location and availability.
Hypos and/or anerys run around $100 and up depending on whether there's reduced patterns and upwards of $200 if they are het for something. $300-400 for albinos and more outrageous morphs like sunglows might be $400-600 and up. Snows are going to be pushing $1000.
BCC's would run higher than normal BCI's. They are harder to come by and many consider their bloodline more 'pure'. The BCC is the "true red tail".
Sounds like you're just looking for a nice pet? You'll love having a boa - they are so inquisitive and personable! You can't go wrong with a normal BCI. You might even find a hypo or anery in your budget. Either way, we'll expect pics once you bring him home!