If you are considering a RTB, you need to consider the cage it'll need as an adult. You could save for a cage. If you're getting a baby, it'll be a long time before a 4' cage is necessary. In my opinion, there is no tub that will be big enough for an adult RTB (either BCI or BCC). It is unfair to a very intelligent snake. Jeff Ronne, the Boaphile, talks about this on his website and it's not because he sells cages - he has racks with tubs too and they can also be pricey. But it is good advice....
A BCI is not a "true" red tail boa (those are BCC) and the BCIs do not get as large. A male common columbian boa (BCI) will be around 6 feet as an adult - much more managable. I still believe it should have a 4' cage as an adult, but 10' is not realistic for a male BCI. A BCC will be bigger, but a male BCC will be around 8-9 feet and the females 10-12. BCIs will average 5-7 for males and 7-9 for females (and that's being generous, lol). There are of course larger and/or smaller specimens of either species, but these are rough averages.