Luxury items tend to have obscene mark-ups in real economies to make up for the lack of turnover of those products. If you make a living selling food, for example, your mark-up doesn't need to be high because you're selling enough of it on a daily basis that you can live off the nickles and dimes that are constantly flowing in.
With luxury items, however, you have to mark them up much higher because you might only make one sale a week... or even a month. Given that one sale has to feed you AND restock your shelves, you can't afford to give the same smaller markup that you'd see for more commonly sold goods.
Not to mention, luxury items aren't cheaply made by the Do It Yourselfer. You can't make yourself a well crafted diamond ring for a fraction of the cost of a jeweler because you simply don't have the specialized skills to do so.
Adventurers don't, in general, have the specialized skills to enchant magic items nor cheaply acquire the materials to do it. If they want something, they can do it at retail by taking the quick and easy method of grabbing a ritual, and casting it. It's quick, easy, versatile, but it's probably not how those in the business do it.
But adventurers don't have the -time- to do it the way the businesses do it, because they're too busy adventuring to take up the trade. This is -realistic.- If the businesses are only paying 1/5 for the adventurer's stuff, this is because 80% markup on magic items -they might not ever find a buyer for- is -very- realistic.
You can't use the economic model for low-cost consumables to simulate the economics of luxury items. Think of it this way.
You go in, you buy a long sword, that's the equivalent economic investment as buying a workable car. It's not a great car. It's just a decent car.
However, you're in the business of using the sword to great purpose. A standard run of the mill economy car just doesn't cut it. Eventually you're going to need a sports car, because you need one that's bigger/faster/stronger/better. You're no longer in the ordinary peasent markets, you're going into the specialized luxury markets. You're making the one sale they'll get for the entire month, maybe even the entire year. They need to eat. You're going to have to -pay-.