Heart Rate Monitors?

I just bought a HR monitor/watch yesterday. I haven't had a chance to try it out yet. It wasn't a real expensive one. It was $28. It has you put in your age and gender. Then you hit a button when you start exercising and your supposed to push the HR button periodically throughout the workout. When the workout is over, it will calculate your calories burned, based on an algorithm. I was just wondering how accurate it really is?