Yep -- about a $40, pay-as-you-go BoostMobile phone, with all of its sounds turned off (and a little piece of black electrical tape over its speaker, just in case), charged up and placed into my son's baseball gear bag (it was offseason) in the trunk of her car.
You can buy an app from BoostMobile called "MapQuest Find Me" for about five bucks per month, and then just add minutes as you need them (you'll hardly need them at all, since you won't be calling the phone or making any calls with it). BINGO -- a sub-$50 GSP unit.
The program is really good. It worked about 90-95% of the time, although I did have to take the phone out and re-charge it every couple of nights, and then get it back into the trunk without my wife catching me. Then I just logged on to a secure website, and I could "see" where her car was (this was legal, as the car was in my name and I paid for it). You can even set e-mail "alerts" when the phone comes within "X" distance of any predetermined location, so, for example, when her car was within 100 yards of OM's house, I'd get an e-mail.
Saves your time and worry with constant snooping, and the screen-shots make GREAT stocking-stuffers for your attorney
