I seriously doubt that you will find a bike computer in the $20-30 range that has Bluetooth, which is needed to connect to your phone and the Strava app. Purchasing a stand alone bike computer will solve the problem of being able to see your speed, distance and cadence (if equipped to do so) but if you want to see your cadence, and possibly your heart rate, on Strava, you will have to manually enter them at the end of your ride. However, it will not graph your cadence or heart rate but will display the min and max values.

If you're not looking at spending a few hundred dollars in order to get the bells and whistles offered on a Garmin and similar products, you may want to look into the Wahoo RFLKT ($79) or RFLKT+ ($99). The RFLKT is not a bike computer but it does connect to your phone and your Strava app. The Strava app sends the RFLKT page layouts (which can be customized) and data collected by Strava (speed, distance, times, etc) is displayed on it's screen. This will allow you to see your ride data without having to keep your phone's screen lit during the ride. If you feel the need to use cadence and heart rate in the future, you can get the Bluetooth sensors and connect them to your phone and the Strava app and that data will also appear on the RFLKT's screen. It's a cheap way to get the bells and whistles of a Garmin, or similar product, without paying the hefty price tag. The difference between the RFLKT and the RFLKT+ is that the plus has a built in altimeter and thermometer and it has an Ant+ bridge that will allow you to connect Ant+ or Bluetooth sensors to an iPhone. The altimeter, thermometer and Ant+ bridge do not currently work with Android phones but some Android phones do have Ant+ support.