Sounds like most people estimate surprisingly well. Fermi-wise, the obscure term in your numerator is 10^10 and the obscure term in your denominator is 10^8. The more obvious term (room size) is 10^1. So the answer is 10^3, and people are guessing 10^0 or 10^1. So some people are only off by a couple order of magnitude, which is explained by a rather small estimation error in either or both obscure terms.