There are several species of "leaf-tailed geckos" (aka the Uroplatus genus) and as such initial investment can span a large range. For instance, what species interests you? Would you prefer to get CB or WC? If a larger species (like fimbriatus), I've know a person that keeps them in custom vivaria 2'x2'x4'! Alot of variables involved, but I'd say $300 to acquire and setup a single Uroplatus ebenaui (wild caught animal). Need a bit more info...
Haroldo nailed it on the head. The ebenaui can be tiny and will fit well in a smaller enclosure. While the Fimbriatus will take quite a large area. Also having never owned one, make sure you dive head first into some internet searches to look into their care. They are amazing animals!