Peruse an airbrush buyer's guide to find a brush with maximum value for your purposes.
Shop around at second-hand stores, pawn shops, discount art supplies stores and other establishments that may sell a used airbrush for cheap.
Buy a single-action airbrush instead of a double-action ones. Single-action brushes sacrifice a bit of control but are a less expensive alternative.
Make airbrush art that is limited in size in order to reduce the use of paint supplies. For example, if you're just getting into airbrushing, start with small paintings and designs rather than larger artistic (or commercial) applications that use a great deal of paint.
Use inexpensive sources of air, such as an air tank or compressed aerosol can.
Rent equipment instead of buying. If you're hesitant to invest a lot of money in airbrushing, or you only need the equipment for a short-term or one-time project, you can rent brushes, compressors and nozzles from arts supply or other stores.