What's the deal on shower fixture/faucet/mixer/valve sets?

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What's the deal on shower fixture/faucet/mixer/valve sets? Why are they so expensive?
I'm working on building a bathroom in an addition to my current home, and am looking for a shower head/faucet/trim/valve/mixer kit that has everything needed to make the hot and the cold water line into a shower. What is the specific name for a kit that includes everything I need? Or should I buy a mixer, and a trim kit with a shower head separately?
Ideally, I'd like to get it for less than 150$, and would like to have the directly facing down rainfall type shower head (due to size of shower and construction) as well as potentially (but not necessary) hose shower head thingy on the side.
Where should I be looking for good deals? What do I need to look for so I know the mixer won't crap out on me after a year, or that the chrome flakes off, or that half of the entire unit is plastic? I'm either finding low end, cheap kits that reek of low quality, or I'm hit with 500$+ kits that don't look like they're worth 500$.
Any help would be appreciated.