Hi Goomba, IMHO if you are wanting to train cheaply with an AR platform, like Yunus says, .22LR is certainly the route to go. canebrake nailed it too, the S&W M&P 15-22 would be my first choice over a dedicated .22LR upper. You get the WHOLE rifle for under $500.00 and from what I've been reading, they are Outstanding!
Your thoughts on the 5.45x39 upper are correct, but If you only want to shoot non-corrosive, or don't already have a large stash of cheap surplus ammo, you won't save much over the cost of 5.56. In my area the corrosive surplus is already 20 cents per rd. by the box - Don't get me wrong, I really like mine, but we are talking CHEAP training here and I am feeding them at a dime per pop which is still not very cheap compared to the .22LR.
I agree 100% on .22 conversion kits, I have the M-261 and have no complaints, but I would much rather use my .22LR CZ-22 dedicated upper, far more accurate. The 9mm upper is lots of fun too, but not much good past 150 yds. and 9mm is kinda pricey.
I personally have not tried the S&W M&P 15-22 yet, I'll let ya know after christmas, that is my present from the wife this year, she just don't know it yet!! Good luck in your choice! and sorry about the old gratuitous pictures, I need to take some new ones!! Ken