Heat it with a hot air gun or hairdryer, it'll push out easier, hold its shape better and stop paint cracking.
Had to do this to mine recently, took a loooong while with a poxy little hairdryer though :/
Before
My guess would be roughly 1 can per wheel. To get the benefits of plasti dip, ie toughness and the ability to peel it off if needs be, it needs to be applied in several coats for a good thick covering.......and I guess that won't come cheap :/