I was recommending refinishing, not touch up. Here it is about $120 all in per wheel (or was the last time I did it.) Agree with Dave, don't go with a mobile touch up.
08 E90 | Jet Black / Black | 6MT
Mods: RAC RG63 w/ Pirelli Corsa 255F/285R | Akrapovic Evo Exhaust | OEM Blacklines
Winter Setup: OEM 18s w/ Nokian Hakkapeliita R 245/40-18