I ended up going with a Viatour Underground tour, which as you say, is close to 3x what Coopculture charges. The extra cost is one thing, but I was actually more upset about not getting the combo underground/belvedere tour, which seems like the best of all worlds!
Oh well, that's what I get for sleeping at midnight instead of staying up and hitting refresh on my browser. Perhaps it's karma coming back at me for getting 2 additional Hamilton tickets when I bought them for my wife. I must confess that I sold the extra 2 for a profit, to a very grateful gentleman who (like me with this tour) completely missed his availability window, and would have been in the doghouse with his wife had he not been able to find tickets. : )