The Retroshift BURD CX was a cyclocross specific rear derailleur 'designed' in muddy ol' Portland, Oregon. The design process involved starting with a respectable microSHIFT derailleur, upgrading its pulley wheels (there was a choice of Shimano Ultegra or FSA units with ceramic bearings) and increasing the tension on the p-pivot spring.
Because cyclocross is hard on parts, it was very important to Retroshift that the resulting derailleur should be inexpensive, and they even went so far as to promise to rebuild any derailleurs for $34.
Just for your information, Retroshift was rechristened 'Gevenalle' in 2014, BURD stands for 'Blatantly Upgraded Rebranded Derailleur' and CX stands for Cyclocross.
It's a likeable piece.