You can start out for free and just load the Strava or RideWithGPS or MapMyRide (or many other) free apps on your phone, start it up when you begin your ride and you will be able to track your mileage, speed, elevation, etc. Start it up and put the phone wherever you normally carry it, or if you want to look at numbers as you ride $20 will get you a handlebar mount for your phone.
If you don't want to use your phone, start out with a cheap simple one like the Cateye ones mentioned - you'll be in the under $75. If you want to get into uploading your rides to an internet site like Strava or RideWithGPS, the Garmin's mentioned above will get you there and keep you under $200.
There's another level of Garmins and Wahoo Element devices with more features, better map routing, etc that puts you above $300.
Last edited by jpescatore; 07-03-20 at 04:19 AM.