Whatever kind you get, you want it to have these specific features for the most accurate calorie burn estimates:
1) Chest strap to measure heart rate. Any version that requires you to push a button or reads only from the wrist is going to be less accurate.
2) Includes a way to enter your gender, weight, and age. These factors are important for accurate calorie burn readings.
I found a cheap Sportline one at Walmart for $50 that served me well for over a year before the battery died. My only complaint about that one was that you couldn't stop it counting, so you had to take note of the amount at the end of the workout. It would continue counting as long as I was in the car, too, even if I had removed the strap! As long as I did that, though, it was fine, and it served my needs. When it died, I upgraded to a more expensive, but far more useful and trustworthy Garmin Forerunner 110. I love it because it's got built-in GPS and I can track maps when I'm outside, and can label activities as running, walking, biking, whatever.