I seem to remember most shops will only do the one off runs for prototypes or just one off keyboards if you are willing to pay like 700-1000+.
I worked in an industrial machine shop.. You're being taken for a ride even @ $150..
I think you both have a point. "Everybody's a hero in aluminum" - it is easy to mill and way cheap. OTOH, setup costs are real, as are marginal costs.
If you're making 1000 cases, absolutely, that should be one hell of a case for $150 each in shop costs. And $700 setup costs for a one-off would crazy. Reality is somewhere in the middle.
My main complaint is that so many folks here don't know what good milling looks like. The surfaces on so many boards look like there wasn't even a finishing pass at all. Inside pockets where a tool change would have made the difference between nice and 'looks like a rough prototype'. Etc.
/me, still trying to get my new mill working well...