You might also, as a guideline, pick a number to represent the most money you ever intend to spend. Photography is the most collectable of all the arts. Photography is the most collectable of all the arts.
Photographs are cheap, plentiful, and available from a broad range of sources.
There is no better way than with photographs to learn to exercise connoisseurship and express your own individual sensibility. The common attitude is that collecting any kind of art is the province of rich dilettantes who have both money and time to burn, not to mention a vast and encyclopaedic knowledge of all things art-historic. I've seen outstanding photography collections put together for next to nothing, by people who started out with no more knowledge than average. Don't collect what other people like, or what everybody already agrees is great. It's been done.