I've had some experience replacing both...
First, you really need to know who you're buying from. If you want to buy just the lamp, it's cheaper for sure. @Dennis, that $91 I'm a bit skeptical that you actually bought an original equipment product. from a bunch of places I've found that the lamp's OEM is Philips Lighting, since they make both lamp and ballast. If you put like an Osram or other third-party or knock-off, they won't last as long. So it is definitely worth it to spend a few more dollars for an OE product in cases like this where life expectancy is an issue.
Second, I think that buying the complete assembly is the best bet. My second go around, I found that buying used cages were bad for my TV! A technician told me that buying an old, recycled cage could work for awhile, but older, recycled cages and housings are really bad because they get warped over time. They also become brittle and break easier. The lamp is supposed to focus the light a certain way and if the housing is warped, it will focus the light beyond tolerance levels, which lead it to take out the color wheel, or another part in a DLP.
So that's what I've learned from this whole mess. I love my DLP, but it's like owning an old BMW and it takes a lot of maintenance. It's worth it for the picture quality, but there's so much you need to know that they didn't tell you at the store! Buy from an authorized dealer, and buy new plastics because older, recycled or reconditioned housing units are just a headache. Take it from me!