Buying a heart rate monitor was one of the best things I did to motivate myself to exercise. There is nothing more rewarding than reading the number of calories burned after each workout! As you get more experienced with it, it can also help you choose the correct intensity for your workout.
You don't have to splurge into some fancy device. If you are a beginner you might as well just purchase one with the basic features from a respectable manufacturer. I have a Polar F6, it cost me about 60 euros (80 USD) and it's worth every cent of it.