I had the same problem with my truck, putting an extra 500.00 worth of tires onto a 1000.00 vehicle didn't seem to make much sense. So I tried running what I had all year round. The truck ended up lasting WAY longer than I thought, so I ended up with a pretty good test.
Well I finally replaced my winter Yokohamas on the front at (IIRC, not at home now) 110,000 km, while the Artic Alpins went 65,000 on the back.
The X-Ice on now have 30,000 km or so, and have lots of tread left.