System evacuation is costly because the government says I have to have a license to buy the refrigerant (in 30lb cylinders) and the EPA can fine me if I don't prove that I'm recovering the stuff, etc. The machine to do this effectively and properly filter the refrigerant so it doesn't contaminate my system is about $6,000. The filters have to be changed, it requires an operator, etc, etc.
Not to mention the overhead of running a shop - insurance, payroll, rent/mortgage, etc. It would make your head spin right off your shoulders.
That's why nearly all shops have a one-hour minimum. By the time I make an appointment, talk to the customer about their concerns and expectations, perform the work, write up the invoice, etc - it's just not possible to not charge for these services.
AC repair is actually pretty simple for the most part. There's no magic to it. For the most part Subaru systems tend to be easier than most. Usually bad Shrader's, bad o-rings, or a bad compressor. Sometimes hose crimps or a hole in a condenser. Subaru's tend to be fine with replacement compressors without touching anything else or flushing anything. The receiver/dryer acts as enough of a filter to keep the debris from plugging things up so typically you just throw in a new or used compressor, a few ounces of oil, some new o-rings, and charge them.
GD