I'll take another route (although probably just as meaningless as most others). The normal way of generating money over the net is through advertisements. How much would Wikipedia make in advertisements, would they use them? Using a conservative price of 0,5 cents per pageview, and using the data from http://stats.wikimedia.org/wikimedia/squids/SquidReportRequests.htm I get that just the value of having Wikipedia for one month equates 1 billion dollars (American style billions).