If you google up precision XT heart rate monitor, you'll find a cheap heart rate monitor sold under several brand names. I have one that works pretty well. It will sometimes give me unrealistic results, but generally works well.
I also have one of the cheaper Timex HRMs. It is more reliable, but doesn't have many features.
Either of these are probably about as good as you can get for less than $50. The next step up (about $120) gets you into HRMs with better communication between the transmitter and watch, like the Garmin FR60 with ANT+.