There is a flaw in your reasoning: you assume that because you can charge a certain amount of money for doing something, you could charge the same amount for doing anything, which seems unlikely. If you are an educated, experienced software developer, for instance, opposed to an encyclopedist, chances are, noone would be willing to pay you as much for writing Wikipedia articles as your clients do for your programming (or whatever).
Thus, if you were only in it for the money, you wouldn't be writing Wikipedia articles—you would work longer hours doing what you normally do instead, making more money and working more (economically) efficiently.
That means, to calculate the economic value of your and others' contributions to Wikipedia, you can't just multiply the numer of hours spent updating Wikipedia with the hourly wage that the contributors would make performing their usual work. Rather, you would have to compute how much it would cost to recreate Wikipedia using paid contributors, which is surely vastly lower.
Whether the result of such a computation would justify placing ads on Wikipedia is another issue altogether, of course.