I am now just doing a radiant system in my house. It is a retro fit so it will be a "staple up" installation between the floor joists. My house has 4 zone heat and 1 of the 4 zones I never use (I have a wood pellet stove in that area). Anyway, radiant heat is not that difficult as long as you understand the principals and theroies behind it.
As far as your question, it is very economical. In a garage, you could use a electric hot water heater, a gas hot water heater or a small tagi heater. You could go as simple or complex as you want. Some things to consider:
1. Very important that all circuits are the same length of tubing. Don't run tubing more than 300' per run.
2. Balance the system for the supply and return. Very important if you want even heat.
3. Get the correct size pump, one that will flow the water fast but no too fast (think radiator with no thermostat)
4. If pouring a slab, secure the tubing with zip ties. Always draw out a plan and make sure the runs are less than 15' difference in length. My uncle did a sunroom with a hot tub in it. When done, all 6 "circuits" were within 1 FOOT of each other in length, now that is planning.
Any other questions, just post up. The basic idea for tubing is to run 8" on center. I believe you can take the square feet, divide that by .75 and you will get a rough idea of how much tubing you need. The tubing isn't that expensive.
Also, if doing a slab install, when pouring the slab, have the tubing pressurized, that way if something happens, you know now and not after the floor has been cured.
Other things to consider, concrete takes time to heat and cool off. While it is just a garage and you might be able to deal wit hthe swings, if you really plan on spending time out there or have a high value car/tools, it would be wise to invest in a computerized controller. These controller aren't cheap, could actually cost moer than anything else you buy for the job. But they are worth every penny. What happens is the controller knows time of day, senses sunrise and sunsets, reads outdoor temp and indoor temp and factors all this into the controll of the floor. The main key is that if the sun is rising and it was 45* yesterday, it will know the precise time to kick on to keep a constant temp. Also as the sun sets, it knows it must kick on earlier because the radiant heat from the sun is gone. I think you can get the picture.
