I use a Garmin Forerunner 301 GPS(101, 201 models will work the same way and are the same size). It is smaller than a smart phone and very accurate. You should be able to find one used on ebay for $50 or less.
just did a quick search and found this.
http://www.ebay.com/itm/Garmin-Forer...item27c08b3e3f