My 22 is either a $600 rifle or a $350 upper for a AR15 lower you have already, depending on how you look at it. I put a $170 Burris 2-7x scope on, which I've really like so far. It's very useful as a trainer, saving money on ammo (it's paid for itself several times over with 5.56 prices), plinker, liberty training rifle, introducing new people to shooting, and just plain having fun.
As others have stated above, it makes perfect sense to pay $500 plus for your 22 rifle if you do most of your shooting with it. I'd say 95% plus of the rounds I put downrange while off-duty are 22LR.