If you restore deleted Items from your Battle.net, you don't have to pay for them in the mailbox. Although my battle.net said: xxx amount of dust and crystals is needed.
I just restored 30 Items, which were disenchanted in my Garrison and I was able to disenchant them again. (I don't have disenchant profession)
So if you disenchant 50 Items and get a temporal crystal ~100g, you can restore those 50 Items again and disenchant them again to get those crystals again.
I just talked with a GM and he told me "It's intended, in the past we restored them for free, too"