You can get a Cateye Strada with cadence for about $35.
http://www.modernbike.com/itemgroup.asp?IGPK=2126175258
Cadance is nice to have you want tp peddle at 70-90 rpm's with a lite pressure on the pedals. It takes pratice but once you get use to it you can go by feel. You can also use a chart from Sheldon Brown's web site to get an idea of your cadence by using your speed.
http://sheldonbrown.com/gears/
I used two inexpensive Schwinn computers from Walmart and made my own Cadence meter. It works really good and once you get the hang of it you can take it off.
http://www.instructables.com/id/Very-Easy-Cadence-Meter-For-Your-Bike-12/