Ugh...everyone's nightmare, that ridiculous "Oops" moment that can happen while backing in a tight space. Sorry.
Guesstimate for sure, but...it looks like the wheel well liner needs to be removed in order to get a tool on the inside of the crease, to gently massage it out.
This might involve jacking up the car and removing the wheel. If the plastic bumper cover is creased, it might not be fixable.
Cost will be based mostly on labor time, which varies from region to region.
(Don't forget that YOU'LL see it every single time you walk up to the car. Almost everyone else won't even notice it.)