Okay, so this is not a publisher, but an author co-op putting together an anthology as a promotional package for themselves and other authors, and to do so, the authors are submitting stories based on their own previously published works and paying $50 to be included in the anthology.
As best I understand things so far.
Now, personally, I'm not really sure what I'm getting for my $50, much less burning a story I could possibly use in a book/collection of my own, without more research to see what kind of reach this anthology is likely to have - how many readers are likely to buy the book and where is the book being sold?
If they have a broad readership beyond what I was likely to reach on my own, then I would see this more as spending $50 and a story as advertising (hence coming out of my own personal advertising funds) for my work. I do not see this as a publishing credit nor do I see this as something I'm likely to see much direct return from.