Projector bulbs, or lamps as they're called by those in the know, are ridiculously expensive. That being said, I wanted to share where I've just started buying replacement lamps from: projectorlampexperts.ca. For many projectors they have not only the OEM lamp, but also a cheaper "alternative lamp". I tried one of those, and it seems to be working just fine. It's easy to find the lamp you're looking for, shipping is free and quick, and they are the cheapest source for projector lamps that I've found.
I sometimes think I should get paid for these endorsements, but perhaps that would cloud my journalistic integrity. I only write about things that I use and would recommend to my friends.