The bare minimum for most adults is 4Lx2Wx1H. This doesn't leave you for any room for any sort of decorations or anything.
You may not want to throw a small BCI in to a cage that size however, unless you plan on filling it with A LOT of decorations so your snake can feel secure. Rubbermaids work great if you want something more simple until your snake grows.
Ball Pythons are generally worth between $100 and $200 depending on age and sex, as well as condition. Some females may be worth more. BCI start at $100 and can go up to around $250. Exceptional specimens may fetch higher prices. You may be getting ripped off if you trade both a ball and corn, all depends. Just use your best judgement