You are in the boulder area I thought, there is no month in which the mean temperature gets below freezing. January being the coldest month and the average daytime high is 45 and the low is 22.oregon only gets down to 35 in the winter. colorado can get well into the negatives.
and the garages i grow in are not so well insulated.
Insulation isn't that expensive either, I assume you already have the garage partitioned off with a room inside for the grow. You probably will be able to get by with nothing more than a space heater, and then only have to run it on only the coldest nights. The lights themselves provide quite a bit of heat, perhaps not venting it all might work.
If you're handy you could insulate the entire garage and add a Nat Gas convection heater for less than $1500.
Or are you just going to hire someone else to do it all for you?
Why didn't you buy a house with a heated garage in the first place? Its a fairly common option in houses within the snow belt these days. Especially seeing as how you are now a business man with a valuable product.