How much does it cost to re-roof a small garage?

It depends on your area, if you need to pay help, if you will do it all yourself, and the quality of the roofing material. (some shingles guarantee to last 10 yrs, 15 yrs or even 20 yrs and all priced accordingly). Put down the materials you need and what it would cost for a helper etc and if you need more tools to complete it yourself. That should give you an idea. Call your home improvement stores for pricing or look around in them. They are happy to help. You may need new tar paper under it, roofing nails, repair some of the boards of the roof or replace them if they are rotted or termite infested. Some things you can not budget too closely because you will not know until you take off the old roof. If you shingle over the existing roof, be sure it is not more than one layer deep already.

Lenora is absolutely correct. Here is a guide.

https://www.homeadvisor.com/cost/roofing/install-a-roof/

Where you live does matter, in my neck of the woods, remove, repair replace would easily be $2,000 - $3,000 on the low end, most is labor and waiting time for County inspectors. Permits need to be pulled.

About four months ago we had a roofer put a new roof on our house and barn. He took off the old shingles on the barn and the house and put new shingles on.....total for the barn was $850 more or less.

It all depends upon where you live and the quality of shingles. It does not cost anything to get bids - Get three bids and references.

whats the cost to replace a single car roof