I built a DEDICATED .22LR trainer because;
a) it's cheap to shoot!
b) dedicated means REAL lower - I can pop the pins, and snap on a 5.56 upper, pop in a magful of 5.56, and be GTG with my "real" AR in under 30 seconds.
c) it doesn't feel like a 'toy.' I built it to closely approximate my other rifles, for realistic manual of arms, weight & heft, and fire controls. It's great for practicing your breathing, hand/eye coordination, and CQB drills on a budget.
ONCE you've got your 5.56 AR (In my case, I have 4 of them), the .22 trainer makes perfect sense.
Eeep. In the current paranoia, I don't think $800 will get you even a reliable USED upper...