The thing about GPS is that it has a lot of scalability.  You can go from as basic as having a lightbar to augment or replace your pass-to-pass marking system on your implement all the way up to having the tractor/vehicle not only steer itself according to a determined path and have automatic section and rate controls to go with it at the top end.

It sounds like you may benefit the most from just having something close to the former of those two setups.

I personally am intrigued by this new product called Wheelman Pro from AgJunction that retails for about $3500.  It can be ordered online and installed by yourself on most vehicles within an hour. I am told it will NOT output a NMEA signal (GPS data standard) to another monitor in the cab.  Nor does it have the ability to do section or rate control but it will steer your tractor automatically and keep track of coverage using a paired Apple or Android phone or tablet.

If you're intimidated by newer technology this may be a little more difficult to become acclimated to but if you're looking for a low-cost solution that will really increase your productivity and efficiency with any implement that your tractor pulls, I think this could be a really great application for that.  It could also be moved to the combine in the harvest season if you use a platform head in small grains.  Click on the link below for more details.

Handsfreefarm.com