Check craigslist or the newspaper for treadmills. My treadmill was purchased from a guy who moved and no longer had room for it. He is also a runner and, like me, used it during the unbearable humidity and when there is ice on the ground, but just plain didn't have room for it in his new house. He spent almost $3000 on it - it is a high end gym quality machine.
I believe I paid $450 for it.
If you buy a used treadmill make sure you ask when it was last used. Treadmills that sit around for ages gather dust that you don't want in your machine; treadmills also need maintenance as far as lubing the belt goes. You can probably figure on having to undo the belt and lube it one way or another but it's best if you can find a machine that hasn't been unused for a long time (the guy who sold me mine had last used it the week prior. =P)
If you are serious about running, and a goal to run a half is definitely serious IMO, I strongly STRONGLY recommend you hold out and look for a high quality machine that is used rather than buying a cheaper "throwaway" model like you will find in most stores. Even the more expensive machines you might find at, say, Dick's Sporting Goods, are not as good as ones that are gym quality. And always ALWAYS look for reviews!
All that said, I own an elliptical that I bought for $400 years and years ago and used the crap out of it before I started running. It still works great. It's probably almost a decade old. It is a throwaway model for sure, no illusions there, but it's been a good little workhorse for me and I still use it to crosstrain on occasion. However when it comes to a treadmill I would never recommend cheaping out. The difference between a good treadmill and a cheap one when you're looking at the high level of training involved for a half-marathon can be the difference between being successful and being hurt.