Personally, I would be willing to pay $5 to $6 for a single 4096x4096 high quality texture set w/ a diffuse, normal (or bump) map, and a rough or gloss map. As for a set of 5, probably $20 to $25, and for a set of 10, $35 to $45.
Let me know if you add any. I would be interested in checking them out.