The cost of magic items and other economics of magic really depend on how magical the setting is. In a rare magic setting, forget hiring mages, you are questing to even find them, and magic items are super rare as well. In a common magic setting with lots of high level casters, it makes sense to have rates to hire a caster and magic items should be common enough that they are feasible to buy. Not sure a single set of guidelines can cover both possibilities, it might be better to discuss them in the context of setting and give two or three options.
Modern games have started to look more and more like online games in terms of the economy of magic. The design goal is that low-level characters get low-level items, medium-level characters get medium-level items, and high-level characters get high-level items. To enforce this you set logarithmic prices. A first level item is 1 gold, second level is 10 gold, third level is 100, etc. A low-level character can't even imagine enough treasure to buy a 1,000,000 gold eighth level item. An eighth level character has a fortune, but they still can't afford the 100,000,000 gold tenth level item.
This works for its purpose -- tons of magic items available, you're still locked to the capabilities you are supposed to have at your level. But it's totally unrealistic and stupidly broken if you try to use it as an actual economic system. For a tabletop game, it's better to handle item availability and not break the economic system by handling it with prices, in my opinion.
Edit: I see that first paragraph was singing to the choir.