for every further charge of non-rechargeable consumables add a potion.Scrolls are twice as valuable as Potions, this is also the standard price for one-time consumables.Potions are always the least possible price.Of course the DMG have a table, which shows the value of a magic item, but it’s always in a range.
The main question remains: How do you determine the price. Or maybe your players wants to buy some simpler magic gear, like a Battle Axe +1 without going through all the trouble of a great adventure. Or, of course, because you, the DM, gave all these enemies some cheap magical items, for whatever reasons. Be it due the numbers of attuneable items (I love the attunement system for the limit, normally) or because they found simply better gear and the old one is not needed anymore. Anyone who plays D&D in campaigns comes to the realization, that the characters will have more gear at some point than they need.
