I agree with the cost split.
I gave a ball park figure of 15-20$ vs the person's regular salary of 10-15$.
That ain't exactly a fair split, if you're saving $30 or more an hour.
People often put up with $10 or $15/hr because the job is full-time or at least steady, it's not always a grind, and there's benefits.
To go work a short cash job, where you know there's no slack time, and you're probably under someone's nose the whole time... $20/hr wouldn't cut it.
A few years ago, we had a certified Diesel marine company rebuild our old Volvo Penta MD-7A. The rebuild cost 5,000$ - at the respected 150$ /hour rate. Less than a month later, the engine failed completely due to improper work. The respected company did not accept the responsibility. We paid for a certified surveyor to examine the issue. He concluded in favor of us. The repair company did not budge. Considering legal costs, we decided to reduce the damages and buy a new Betamarine engine (best thing we ever did, but that is not the point).
One month before I arrived for vacation this last summer, my father had a reputable marine battery company, five minutes away from the dock, replace battery with an identical one. Labor was 150$.
From what you've said, the Volvo story sounds like a horrorshow. If it was me, I'd have pursued a settlement.
I got to see the other side of this this spring. I can recall a couple situations where people asked my boss to nurse along some ancient, clapped-out engine. If the thing was not rebuildable, or too iffy to be able to guarantee a rebuild, my boss would flat-out refuse, and suggest a repower. Even then, some owners still preferred to have my boss put in time (with NO guarantee it would last, stated upfront) to coax whatever improvement we could out of the old anchor. People are funny...
Re your battery change and $150 charge: if someone's gotta load a battery onto a truck, drive to a marina, unload the battery onto a cart, go to the boat, haul out the old battery, install the new, test new, confirm it's charging from both the alternator and shorepower, haul old battery back to truck, load truck, drive back... that's easily over half an hour. Maybe even more than an hour.
That's also not taking into account: wait for owner with key... always fun

, replace a lug cos it's shot, maybe diagnose why the old battery went (if it wasn't age). Some boats are utter backbreakers when it comes to battery position.
I just don't base value on labor simply because someone hung a shingle out - saying he is a pro and knows what he or she is doing. Of course there is good and bad. But the shingle and the high prices are no guarantee of quality.
Low-balling, on the other hand, is as near a guarantee as you can find for unsatisfactory results.
I do hear your frustration, and I am sympathetic... to a point. Most people need to have made more then $10 or $15 an hour, in order to keep a boat. How do you justify your high rate?