Water/Air Temp guage

Hello, i have a small 16' bass boat, it has alot (not all) of the bells and whistles, but one item it lacks is an air/water temperature guage.  I have a 1978 monark bass boat with a 55 horse evinrude, it has 2 fish finders, one a hummingbird (which does not have a guage) and an eagle by Lowrance (which truely shows its age).  The trolling motor is a "scout" also by evinrude.  Has anyone bought and added an air/water temperature guage that is worth the money/affordable?  I saw the ones for minn kota at Wal-mart this morning but it looks as if the guage plugged into a minn kota trolling motor.  Are any of the ones in basspro, academy, or gander mountain worth the money (and time and effort to acquire them).  Any help is appreciated.  

Bassguy6

P.S. Any price range is acceptable (unless suggestion is buying a new fish finder).  I am a college student and I work construction so I am about to get a tax refund check! hehe