Art is always a tricky thing, because none of us small timers have the budget of WoTC or larger companies (although I think Morrus has a much bigger budget than most due to the success of previous projects, but that's speculation--I don't see his ledger or budget lol. Maybe he spends it all on blood sausage and tea).
It by far is the biggest line item for me, historically. A project like Level UP would probably need an art budget of close to $10,000-$20,000 or more in order to compete with the big boys. I don't know many Indie publishers who can do that up front.
Right now on my current project, Chromatic Dungeons, I've already spent about $5000 on art, and that's mostly black and white interior (not full color interior), and I've used stock art as well.
I only mention all of this so as to put the art samples into context. If you're judging or comparing it to WoTC, I don't know if that's fair because I doubt the budget is there for that.