I got a 120 GB drive from eBay for around $70. Works great. Two caveats, though:
1. In order to transfer all your data properly, you'll need a transfer cable. You can get one for $8 or something from Microsoft. Do NOT buy one from eBay; I made that mistake and it was not the official MS cable and did not come with the data CD that is required to make it work. I just called Xbox Live Support and they were able to help me.
2. I didn't find this out until a couple weeks ago when I got a couple of original Xbox games, but the new drive I got doesn't have backwards compatibility. It might be possible to add BC support, but the guides I came across were a bit complex and required even more gear I didn't have. Turns out it works just fine if you just swap out the new hard drive for your original one when you want to play an older Xbox game. So hold onto your old drive if you go the eBay route.