A $500 Trimble is still the same 12channel + WAAS unit you have in your phone or buy in a sports store.
A Real Time Kinematic system can do mm (on a good day with a following wind) but they cost 20x as much!
My post wasn't totally clear - I meant people ALSO wonder why an RTK is $10,000 when you can build your own for a lot less (http://blog.makezine.com/2009/11/12/diy-real-time-kinematic-...)
