I haven't had mine put in yet, but I just got 7 quotes for my garage. It should at least give you a start for some pricing.
My garage is about 1100 square feet in an L shape and I needed about 100ft of gas line to get to the heater. I was looking for a 75,000 btu unit. Some people were quoting Lennox, Modine, and another brand I've never heard of. Here are the quotes I got.
1943
1500
1500
1800-2000
2200
1625.97
1800
I ended up calling one of the Lennox dealers who came in at $1500 and asked him if he'd be interested in doing it for $1300. He said he'd do it for $1350. I was pretty happy with that because he's the one that installed my Lennox home furnace when the house was built.