Is the garage attached or detached?
Is the garage tied into existing power, water, and sewage? Usually a sewage tap is a big-ticket item.
What's the design of the garage cum ADU? Different designs cost different amounts to pull off.
What are the building and permitting requirements of the area the garage is in?
I know you want a ballpark figure, but it's really tough to give you anything useful. This is something you should ask a local contractor, preferably one with a lot of experience doing this in your area.