There are three system components to GPS: the system, the receiver and the maps. The system (satellites and timing) are VERY accurate; the receivers vary in how they process the data (generally, the more expensive receivers are very accurate and process data quickly), and; the maps (supplied by commercial vendors whose accuracy varies all over the map!).
I was involved in developing (30 years ago) and commercializing (25 years ago) GPS and the first receiver I bought (on the Government dime) cost $250,000+ dollars (1992 dollars). Today, you can get similar performance for $8,000-$10,000. Basically, you are paying for quick signal acquisition and processing time.
