I don't have significant issues with road debris, so I'm running just standard Marathons. I got them for $24 each and have about 5800 miles on them. The front still looks almost new, the rear looks like it's about half-worn. I think I've only had one flat with them so far (about 1.5 years on the bike).

Assuming things work as I expect, after about 2 years I'll put the front on the back and buy another one for the front. Normally rotating doesn't make much sense, but the way they're wearing I think I might have the same tire on the front for 10 years, and at that age I'd be worried about rot. I figure better to move it to the back and wear it out, rather than wearing out a new one.

At this rate I'm paying about $12/year for tires.