The trouble with a poll like this is what exactly you mean by going single. For some people you just slap on a cheapy turbo, manifold and simple airflow fudger and that's a single setup, doesn't matter if it doesn't really run that well, has a dodgy cold start etc etc. For others it means optimising the potential with cams, a decent turbo, sufficient cooling and more importantly mapping properly with correct ignition timing etc. And there are those who would insist on engine internals etc too.
Personally I don't see the point in simply bolting a turbo on, I want it to be drivable every day to and from work, and also survive track days. I also want boost to come in very early, and I want to get the most out of it as possible in terms of optimised fueling and timing. I also need it to be reliable. So I guess I'm at the top end of the second camp above.
Also, I fitted mine myself, and sourced all the components myself because none of the kits fitted my requirements exactly. So there were no labour charges for that.