The Drotek XL RTK GPS receiver is a competitively priced RTK GPS module that can be configured as either base or rover. Using RTK, PX4 can get its position with centimetre-level accuracy, which is much more accurate than can be provided by a normal GPS.
Drotek XL RTK (drotek.com)
RTK setup and use on PX4 via QGroundControl is largely plug and play (see RTK GPS for more information).
The Drotek UCenter tool is not required for configuration - everything is handled by QGroundControl.
The module inputs/outputs are shown here (Drotek docs)