Depends, the price can go up but it can also go down, that was a smart one ehh. I expect it to go up along with all the other rares in a period of 2 years, but who knows. It all depends on coming updates etc. But you could give it a shot, I guess.
However, if you've got quite some mills and you really have no further interest in Runescape I would just sell it for Irl money.