I don't think mSATA is being used in new notebook PCs anymore, having been supplanted by M.2. That means there are now relatively few[er] suppliers of mSATA drives. It also means they have little reason to discount these drives. I suspect the larger retailers like Best Buy and Newegg have already cleared out most of their mSATA inventory.
So your best bet is to look for smaller retailers who are trying to rid themselves of remaining inventory. The best advice is to just keep looking, know how much you're prepared to pay and then pounce on deals if/when you see them.