BG Motorsport are at Silverstone and come highly recommended from pretty much everyone who's used them. There are ever so slightly cheaper places, but apparently BG Motorsport offer one of the quickest turn around and include damper balancing on a dyno.
Needless to say cost can vary a lot as it depends on make/model of damper, if any abnormal parts (besides the usual seals etc) are required.
Bottom line is refurbs aren't cheap as parts aside, the equipment & know-how needed isn't particularly commonplace. The Trophy Sachs are over £200 per individual damper to refurb for example.
If they're budget coilovers this could be a perfect excuse to upgrade?