A "site" or any business is obviously worth whatever someone is willing to pay for it. The pizza.com example is one where the "business" was non-existant, but happened to own an "asset" -- ie: their url, which was very valuable. It's like that shabby, rat-infested slum of a house that sold for over 2.5 million in Toronto a few years back --- because it was sitting on prime lot downtown with giant condos on either side.... if you moved that house a couple miles away it would be worth 100K.
Unless some unrelated term (or product, or whatever) also called "merb" gets very popular, then this website will never be valued over a few thousand bucks for the domain name alone. I think this is not likely to happen.
As opposed to "pizza.com", merb is an actual business with actual business metrics that a potential buyer can evaluate using common business principals such as the very basic ones outlined by Fly88. IF (and I'm sure it's big if) someone was looking to buy merb -- I am sure that the valuation and final price would be loosely based on these metrics (reg'd users, monthly page views, advertising revenue, net income etc etc).
I agree with Fly88's estimate and would (with the limited information available) set a vaule of between 200K to 300K. Due to the "risky" nature of the sex trade in general, this business probably comes with more risks than upside, so I am sure if there was a buyer, this would be an important concern.
BD