Before I went into business about twelvce years ago I too used to baulk at the charges for servicing a car. They used to charge around £11 an hour service charge and all the mechanics I knew were earning about £3 an hour.
Once I became self employed I then realised that this £11 per hour had to pay for all the overheads of the business, not just the mechanics wages.
So you can't really make a comparison with a main dealer operating from probably state of the art premises, reception, spares, sales and mechanical staff to pay for, with an independant garage probably working from a hundred quid a week back street garage.
It's the same as a carpet cleaning business, a one man band working from home can operate on a lot lower turnover than an operation with staff and premises.