Okay. What is your in-world justification for why players are for some reason considerably worse at crafting than some first or third level adept, no matter how powerful they become? That makes no sense. It's going to jar immersion hard.
They are just as good at it, or better. How good you are at it depends on your spellcraft skill.
A third-level adept will likely have no more that a +7 or +8 in spellcraft, or +10 or +11 assuming the skill focus feat. So items that have a DC of 22 or more to make are probably out of his league. And a lot of items will fall under that category. Major items are going to require high level casters to make, whether they are PCs or NPCs.
How much it costs an NPC to make a magic item need never actually come up in the game. But a Magic Crafting Laboratory would (and should) be an expensive operation to maintain. If players are able to buy everything they need and craft stuff in their room at the inn with no overhead at all, that jars immersion.