These would be the same people that will tell you they can heat their house and shop for free with wood......
My definition of heating for free is no out of pocket expense, with savings by burning wood that eliminated what I was spending to heat my house with oil.
Well except for some of my time and a little gas and oil for the saws and trucks, actually I do heat my house, 3000 sg ft., free with wood, let me explain. 3 years ago when heating oil was near 4.00 a gal I decided it was time to install a outside wood boiler to heat my house. I went through about 700 gal of oil a year, so the time was right to buy a unit and have a short payback period. My neighbor sells them so I scored a complete unit for 5K and installed it myself. I also added the heat exchanger for hot water. Electric bill in SW VA before installation 95.00 per month (my wife does the wash with hot water) after the installation 45-50 per month Oct-April, so that is a 350.00 savings per year. Year one heat 0 gals oil for a savings at least 2450.00 (700 gal X 3.50) Year 2 was the worse winter we have had here in the 36 years I have lived in these mountains, we had snow on the ground from early Dec to early April, temps in Jan -Feb rarely went over 20 degrees, oil used 0, I would say we would have burned at least 1000 gal of oil so add another 3500.00 in oil, total so far is 700.00 savings for electric, and 5950 in oil. Year 3 which is this year was mild as compared to last year, but it would have been a 400-500 gal year, oil used 0 gals, electric saved 350, oil saved at least 1500.00, so far 7800.00 saved with an investment of 5000.00, plus the very minor amt of elec needed to run the circulator pump, which when you look at the electric bill is taken care of with the savings by not having the water heater turned on. I took about 1000.00 of the savings from using oil and built a very nice wood shed near the stove this past summer.
Now I do cut my own wood off my property so I have labor and gas involved which is minor, living on a farm I already had the chain saws and 4 wheel drive pick up trucks, and being retired I have the time and really do need the exercise, Plus my money isn't going to the arabs to buy a gold plated car!!
BTW the wood getting vehicles are farm vehicles taken off the road long ago, 1984 ford 4X4, 1968 Ford 4X4 and a 1990 Ford 4X4, so I am not using a 40K PU to do this, the chain saws a Stihl 034 & 360 were here long before the stove and were need for other things
Phase 2 of this deal is to extend the hot water and return pipes out to my 40 X 60 garage and heat that when I need to with the wood boiler, the cost of the materials will be about 800- 1000.00 to do this, which is still using money already saved
and as to an estimate to build the garage, ask a bunch of contractors for a price, find out if local ordnances allow you to do it yourself if you have the talent, hit up some of your buddies that work in the trade, ask around for names of a good cement finisher, price out your materials using the bid room or the contractor desk at Lowes, HD and other building supply places. ASK a ton of questions if you do not have experience doing this, decide if you want to add a lift in the future and might need vaulted ceilings, how many amps for electric, do you want water, insulation, finished interor walls etc. etc., the devil is in the details, and the most important thing is that when you are finished you are happy with what you have at a price that you can live with.