I've thought about doing it, but it's just too expensive right now over here. Depends on the size and what your keeping in the refugium. If it were small with macroalgae, a strip of 10 blue LEDs would work. If it were a large sump with macroalgae, xenia, clams, etc., than you would probably need at least 2X50 clusters (one white and one blue) which can be pretty pricy.
LEDs are the future of reef lighting, but costs are still too high to make enough light for a whole tank.