Selling keys reflects key price

Found a bug where if a doors price was set lower then the amount you get back when selling you could get infinite money.
This change uses the same hook that gets the door/vehicle cost to figure out how much to sell it for, instead of the global door/vehicle cost.
Merge pull request #1636 from SuperHighVoltage/patch-1

Selling keys reflects key price
Merged FPP
Merge branch 'master' into workshop