Maxperson
Morkus from Orkus
I think you are missing my point about how the pricing of magic items is unlike, say, a rule for grappling or the mechanics of a spell. You can test the latter by saying, "Hey let's have a mock combat and see how grappling works and test Witch Bolt." But how do you do that with magic item prices? "Ok, I'm going to give you each 100,000 gold and 10th level characters. Let's pretend you're in a magic shop and haggle over prices."
The problem isn't the playtesting, but rather that the game doesn't take magic items into consideration. As pure bonus, being able to buy whatever you want would break the game very quickly.
To playtest, though, you just arbitrarily pick numbers. 1,000gp for common, 10,000gp for uncommon, 25,000gp for rare, and 50,000gp for very rare. Then give the 10th level party 100,000gp and say buy whatever you want. They play it and see if they think that amount of gold and those prices break the game. If yes, tweak the prices upwards. If no, tweak them downwards. You do that for each level with each playtest group and eventually you'd have opinion that you could get the aggregate of and would be useful. However, as stated above, 5e isn't designed to make that feasible like 3e and 4e were.