I've got a pair of Sennheiser HD-25 SP-II headphones (they are monitor headphones and spend a fair amount of time whacked into the front of a Korg Trinity). I paid 30 GBP for them in 2004. I've replaced the cable once for 20GBP after repeatedly running over it with a desk chair for 5 years and replaced the earpads once for about 10GBP.

I would not swap them for anything.

They run rings around headphones which are 5-10x the cost. Rumor has it they piss on 1KGBP headphones. They sound much better than HD280s as well (I've had some as well). The frequency response is pretty much 100% linear. They are very comfortable. They block out pretty much everything. Most importantly you just cannot hear any static/noise on them at all.

They are godlike.

Unfortunately they change hands for a hell of a lot of money now otherwise I'd grab another pair :(