Since retrofitting stuff is mostly about time and not materials, I'd have to guess less than a kilobuck unless you are in a very high cost of living area.

I would agree with ionized. Provided the receptacle/circuit you're planning to tap into isn't otherwise restricted (kitchen area, laundry, etc...), it should be small $ add some lights and receptacles. Note you will need groundfault for garage receptacles.

Ron did bring up a possible wrench in the works that I had not though of. To explain further, the existing garage circuit might be on a restricted circuit in current code version. If sharing a garage outlet was allowable when the house was built or the outlet circuit installed, it might not be anymore and I imagine that you won't be allowed to add to it in that case. That would mean a new circuit from a panel.