Is $35 for 25 2048 ground materials a good price?

For the ones shown, they look incredibly repetitive. Consider for a moment the competition you have, GameTextures, Quixel Suite etc have very highly-polished materials all PBR ready and are very adjustable.

Trying not to be too harsh but my critiques are that they are way too blurry (most look like they’ve been up-scaled just to get to 2048 resolution), very obvious tiling artefacts, and some also seem to have pre-baked shadows and/or light and dark spots in the diffuse map (big no no!). I’m assuming a lot of these have been generated from Photos found either online or pieced together from source art in Photoshop? While that can work for small things that you rarely notice, and what I’d call “filler art”, for centerpiece textures that you’re trying to sell (and for $35!) that’s not something you can get away with.

There is often little you can do without tiling (although the seams in yours are very apparent in some cases), you may want to consider selling Material sets rather than just textures. Work on your shaders a bit more so that they can be used in a variety of ways, and not just as source-art textures for people to make better materials with. Layering differently-scaled materials on top of one another, vertex-paint support for details etc are all viable options. Right now though, there are much more superior solutions already out there which are very flexible (and cheap!).