In my humble opinion, you are better off kitting out a reloading setup than buying an upper in 5.45. Again, in my opinion and from what I have read about the ballistic effects and potential of the 5.45 vs the 5.56, 5.56 comes out the clear winner. If you have a reloading setup you can enjoy the fruits of cheap plinking ammo while still being perfectly able to practice with and load more deadly rounds (75gr hornady TAP etc) for uses other than plinking.

A reloading setup will probably run you around $300-400, so it will be cheaper than the dedicated upper while allowing you to load cheap ammo for *all* your guns.

My $0.02