Most painting estimates are one number and a sentence. That is not enough information to compare two contractors, and it is not enough to hold anyone to anything later.
A useful estimate names the products by line and sheen, states the number of coats, describes the preparation in specific terms rather than as the word 'prep', and carries a start and a finish date. If those four things are missing, the number at the bottom is not comparable to anyone else's number.
The preparation line is the one that matters most, because it is the part of the work that becomes invisible the moment the finish coat goes on. Ask what happens to bare wood, to patched drywall, to glossy existing trim, and to failing exterior paint. The answers should be specific.