I just recently finished my review of the FOBO motorcycle tire pressure monitoring system (TPMS) for our friends over at webBikeWorld. This inexpensive ($99.00 USD) system works with one's existing Android or Apple smart phone (or tablet) which acts as the interface and display. This helps keep the costs down for those running a relatively modern Android or iOS device (Windows phones are not supported).
The system actually works very well and provides quite a lot more options than expected for its price range including sharing of the app for multiple users, psi AND tempertaure readouts, multilevel alerts for pressure over/under limits, etc. If one already has an Android or iOS mobile device this is a great way to keep up with your tires' pressure and be alerted if they need attention.