The cost depends on the company and their attention to detail. There are several ways you can go with it. Just a few ideas to play with;
Go two tone for one. Less paint and body work.
Check out the local trade/tech schools and get them to do it as a class assignment. I had a 1951 Chevrolet done for the cost of paint and a case of beer. (yes, this was several years ago)
Also, check out the local collages/design class. Offer a cash prize for a paint design for your Hummer. Students always need extra money.
Check on getting a decal for the sides. Black heckerboard, about 2" high and a taxi sign for the top.

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2006 Slate Blue Metallic H3, FlowMaster exhaust, True Flow cold air intake system, Ground Force rear lowering kit, custom locking fuel door, chrome hood grill w/handle, US Speedo gauge kit (chrome and red numbers), Custom rear tire cover, chrome tail light guard, custom paint on skid plate, new pin stripping, USMC front license plate.

Semper Fi