You are paying for the solids in the paint. If you buy cheap paint, you have to buy more to get the coverage. Some jobs require more than one coat of primer. Buy the best primer you can afford, and tint it if you have to. Even primer won't stick to greasy surfaces. Clean surfaces with the old-fashioned TSP and bleach solution then rinse. Remove all pieces of loose paint and sand smooth if necessary.
Recommendations: Behr paint and Zinsser primer.

PW