/sci/ - Science & Math

Is there some sort of budget math chart ? Most of the books in the /sci/ wiki cost more than 60 bucks, but I can only afford about 35$ at most per book.
Dover usually does the job, but I don't know what books from their catalog are the best.
I'm personally looking for advanced undergraduate level stuff and beyond, particularity topology, group theory, analysis, logic and combinatorics.
And yes I know I can download them from libgen, but I just can't focus on digital format. I tried, I really did, but I just can't.
So if you have any suggestion, please share.