I have worked with Yuri Dorn at JHRG (jhrg@...)multiple times. They have found records for multiple sides of my family from all over the Belarus area. Costs are extremely reasonable, however I would caution you about the turn around time for research can run into several months. I believe this is understandable given the nature of the bureaucracy they have to deal with in obtaining records from the archives. 
Good luck!

Jonathan Sher