I was you a few years ago, seeing no reason to spend more than $20 on a bike co. Then I got a used Garmin Edge for $100, and it was a good investment. Here's why:

1. You only have to put one thing on your bike. No wires, no spoke thing, no fork thing, etc.

2. You will enjoy getting more data from your ride, and you may end up riding more because of it. You will have an automatic record of all your riding, and you can set goals and see how you do in relation to them. The heart rate thing is fun, too.

Any investment that gets you riding more and enjoying it more is a good one.