Let it be fixed at 10 legendary shards which is their initial cost.
With the cores not being the endgame of armor masterworking anymore, it doesn't make any sense to have this restriction, not even the new ones at banshee have this ridiculous price increase.
Also, banshee could start selling them as well, just for practicality, since he already sells the other 2.
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‘They’re easy to get though!’. So it would affect you negatively how if you could buy them for 10 shards a piece?