Spray painting is not automatically more expensive, or cheaper, than brushing or rolling. On a large open surface, spraying can take less time. Where floors, furniture, windows and edges have to be masked, the preparation can take longer than the painting. The quote compares those two kinds of time for the actual room or elevation.
The other factors are the same as any painting quote: the condition of the surface, the colour change, access, and how many coats are needed. There is no published price difference between the methods, and no claim here that spraying saves a set percentage.
Say which result you want and what has to stay clean. The method follows from that.