May have to do with how often you wear them. For a few years I wore jeans every damn day, unless there was a funeral or something, and I had a similar rotation: two pairs "active" at a time (with maybe one reserve pair in case the lesser of those two got ruined, assuming I had any older ones that were still wearable)—one for light duty, one for heavy duty, order a new pair of 501s when they're on sale about once a year, they become the new light-duty pair, everything else shifts down, oldest become rags or get thrown out (they were never in good-enough shape to donate, by the time they were leaving my closet).
~$40/yr wasn't so bad.
[EDIT] Oh, and for the record, I minimized washing, though I wasn't as hardcore about it as some people who claim to go months between washes. Every wash seemed to age them a couple weeks worth of wearing.
These days, I wear a lot more wool, linen, and cotton/linen blends for light-duty, and wear a single pair of Prana Zions (very thin mid-tier hiking pants, synthetic material) for anything risky/dirty. The Zions are probably at something like 1,000 wears (for the first couple years I wore them kinda like I had my jeans before—for everything) over five years or so. About a penny per wear. Finally due to be replaced, starting to get enough holes that they're getting iffy for wearing around... people, at all. If I hadn't damaged them with sharp things a few times, they'd probably go another 500+ wears, they're not getting worn-looking anywhere, no stains have proven permanent, nor is any part of them fraying (except at the holes).