I just ordered one of the TST tire pressure and temp monitoring systems from these guys. I have one of their older systems that does not use replaceable batteries and the battery on one is dead. Went to buy another set of senders and found could get the new entire two wheel system for $99 shipped. It is also a little neater design but big plus is replaceable battery in sender. The old system worked flawlessly until the sender battery quit. Being able to set a temp and pressure alarm and then not worry ( as long as no alarms go off) is easily worth $100.
http://www.tsttruck.com/Product_List.html