The one consumable much harder to replace is the oleophobic coating. There are drops you can buy and apply, they seem to work alright, but don't last nearly as long as the factory coating.
By year two, the screen just gets grubbier and grubbier. By year three its just plan nasty.
If you get AppleCare+ the screen replacement is $29. I usually get it replaced right before I can hand it off to my family. It's not as good a deal obviously if you don't plan on getting AppleCare+ anyways.
I buy them in packs of 3-5 on ebay for $5-10. (no need to buy the really expensive brands) They are quick to replace, have a nice oleophobic coating on them, and can be replaced as frequently as desired.
By year two, the screen just gets grubbier and grubbier. By year three its just plan nasty.