If you can rent a tractor or backhoe or skidsteer and tear down the detached garage yourself you can do so for the cost of the tractor rental and a days work.
Plus you'll need a dumpster to haul off any debris you have when you tear the garage down.
If you hire someone to tear down the detached garage it could cost $1,000.00 to $4,000.00 including the labor and dumpster rental.
It can vary from demolition company to demolition company.
When I had an old detached garage to tear down I called and got estimates and they came in the range of $1500.00 and $3500.00
I decided to rent a skid steer and dumpster and had it done in a day and paid $200.00 for the dumpster and $100.00 for the day for the skidsteer rental.
I had lots of fun and had the garage tore down and hauled away on my own within 4 hours of fun work with the skid steer.