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davidcyclebackNews services sometimes made photos with earlier images (ala 1950 photo showing Ruth in 1919). Though they are not original and as valuable, they were official and made in limited numbers.
Due in part to his early Death, Mathewson is scarce photo wise. An old but later generation press photo of Mathewson would have some value. If the photo is indeed fromm the 1930s and the image is nice, $100 off hand sounds like a buyable price. It's important, though, to be confident that the photo is from the period mentioned. Meaning, if it turns out to be from the 1960s, it would be worth less. Though if you aren't certain of the age but the photo has an official stamp or caption on back you can at least be confident the photo is official.
For perspective, I once sold a 1950s-60s UPI photo of Mathewson pitching in the 1910s (with official UPI stamp on back) for $50. That's a comment on the scarcity of Mathewson photos and that even much later news service photos are still collectable. A high quality original 1900s or 1910s news service photo of Mathewson in a MastroNet auction will often sell in the high hundreds or over one thousand dollars.