IMO it would be a money sink for the game, but I don't think it would be fair that the multi millionaires would just purchase 50+ rerolls and keep spamming it until they got absurdly high IVs. They could do the same without this suggestion, but the chances wouldn't be nowhere as high as if this got implemented. I'm not sure this is very balanced for a MMO, although I understand the frustration of rerolling a legendary nonstop and not getting anything.
-1 from me for now.
(Also I wanted to delay this post because this is my post number 1000 in the forums but I feel too lazy to wait so here you go)