I have a Polar hrm and really like it, it retailed for over $100 though. But I do see some of the models come up on Craigslist around here in the $50-60 range so that might be worth looking into at least. I agree about getting a chest strap, definitely a must have, even if you have to wait a bit longer to afford it!
I am actually purchasing one from Target today: the New Balance N4 Pearl. It comes with a chest strap and I've read lots of great reviews on it. It's at Target for $69.99.
I have the most basic polar model. I think I paid about $40 for it 3-4 years ago and it's still going. It has a chest strap, tells me my hr, the average hr of my workout and not much else. But this is all I really need.
It makes a HUGE difference in my running. I really struggle with pacing myself so the HRM helps me to not go out too fast.