It's common to pay by the patch, where a batch can be almost anything depending on the project - several buildings, or just the tiles required to put together a bunch of similar buildings. Or, if each building is large, complex, and bespoke (not made up of smaller tiles), then paying by the building might be acceptable, since each one is a lot of work. Artists often give discounts when they can expect a continuous stream of work though, so being clear and up-front about how many buildings you need and when.
The price depends a lot on the size and complexity of the art (plus additional considerations such as whether it needs to fit an established style, etc), so you'll need to provide that information before anyone can give you a useful estimate. A tiny and cartoony sprite takes much less work than a large or complex one, and the prices differ accordingly.