How long should pedals last?
Increase your budget until you can afford second hand pedals with cup-and-cone bearings, then spend a few bucks servicing them.

DMR's V8 and V12 models are very popular and have been the same for years. The V12 typically has better seals and lighter materials than the V8, but both are serviceable.

Here's a guide on servicing the DMR V8:
http://steveukmtb.wordpress.com/dmr-v8-pedal-overhaul-cup-and-cone/

I got Wellgo's V8 clones via eBay for 7GBP delivered (about 10USD?). I don't know if they're the same internally but they might be an option if you cant get pre-loved DMRs.