Bach wasn't relatively poor - he was comfortably middle class, as it was then, with a secure job and extra income from a concert series he set up. (One of the first in the world.)

Mozart was relatively poor because he was terrible with money. Some of his contemporaries, especially opera composers, did very nicely indeed - something which continued to be true for centuries.

Handel was easily the equivalent of a modern multimillionaire. Some performers and/or composers - Paganini, Liszt, various opera singers - earned far more for each performance than modern artists do.