For a start, the fixed price annuals are actually fixed price inspections - they don't actually cover the cost of fixing/rectifying any problems that the inspections show up.
Here in Oz, I suspect most owners simply hand the keys over to the LAME at annual time and say - "fix it".
I think a lot more owners in the USA are more "hands-on" in respect of having a say in what gets done at the annual, especially with the advent of companies (like one beginning with "Sav...") that specialize in helping owners manage the maintenance of aircraft. Could also be that the FAA system of regs does help as well but that's a whole different thread...
Anyway, over there, after the inspection is done, the owner gets a phone call or written quote for the rectification work that needs done and he then says yea or nay to each item, with or without assistance of said organisation...
So really, while the fixed price inspection does sound cheaper, in reality I'd say most of the time folk would pay more than the advertised price, unless of course you took your plane away from the shop and got someone else to fix it (which under the FAA regs you are apparently allowed to do...)
Factor in the tyranny of distance to get a part from Wichita, KS to Perth, WA and you see why it's all so much more expensive...