Both Wahoo and Garmin make a cadence sensor that is both Ant+ and Bluetooth enabled for < $40. With either of those and a app on your smart phone you can connect to it and record your cadence along with the app using your phones GPS to record your track, speed and distance so you can look at it later. Or even while cycling if your phone shows well enough in the bright sun and you don't mind the extra battery drain.

The Ant+ or Bluetooth will allow you to use that sensor with most any of the common GPS/Cyclometer makers should you later decide to pony up the money for a decent head unit. IE. Garmin or Wahoo.

Some of your more inexpensive wireless sensors might depend on a proprietary standard that limits you to always using that particular brand. Which being the inexpensive brand might break or quit in a year or two. So buying better quality can be the difference between 12 or more years of use vs just a couple years of use before it breaks.