I painted the interior of a Coolermaster HAF 932 for a case mod. I bought one can of black primer and one can of flat black paint . I sprayed two light coats of each and I still have primer and paint left over.
How much you need actually depends on the size of the cans. I purchased cans of Rustoleum spray paint at my local hardware store. There are smaller cans of spray paint made for modelers available at hobby shops but they tend to be expensive.
Make sure all plastic parts are removed from the interior of the case, including those tool free plastic hard drive fasteners and video card fasteners. The spray paint for use with metal will eat into plastic. If you are going to paint plastic parts, then get paint designed for plastic.