30 minutes to change springs on both doors? Extension springs, yes. Not torsion springs.
Look at those pics again. 2014 Ford Mustang is over 4'7". Those are at least 9ft tall doors. Big. The headshafts will be 10ft up.
Average 2.5lbs/sq-ft for steel, polyurythane insulated doors, that 9x9 is 202lbs, with TorqueForce D400-12 drums, a .283 x 2"x 49" long spring balances that door. That's not a stock, premade spring you inventory. That's a big *** residential spring. Using the same math, the 16ft door needs TWO .262 x 38" springs.
No one guy could replace those springs on 9 foot tall doors in 30mins. That's tools/ladder off the truck, into the garage, dismantle, reassemble, TWICE, and clean-up in 30 minutes. Not to mention, size, cut and cone, at least three springs, and travel to/from the site.
Unless, you're a one man company, working out of a pickup, and you don't mark up the parts you bought from a supplier or charge for your travel time to size the customer's springs and source them, and you're only working for your wages because you don't have a cent of overhead, $600 is not a lot of money for that job.
I'll reiterate. Those are very large doors. $600 is probably fair for a reputable company that charges $80/hr, who will show up with a service truck stocked with 12ft lengths of various spring sizes, measure the old springs, cut, cone, and install, three springs, then quickly swap out two stripped drive gears, and reset limits on the machines.