Interesting experiment. You raise some good points, though I'm not really sure you can compare the two.
For one, cons aren't only about exposure. They're about networking. Iguess that's pretty much obvious, and I think you even mentioned that in the first article, so I'll skim over that.
But: A conversation with a potential fan may be much more effective in selling your comic than an ad that results in a cursory glance over your site.
Also: How cost-effective a con is largely depends on what you have to sell. For me, they didn't start paying off until I moved from €3ish booklets to €10 albums. And even those can't beat original art.
What I'd like to see in addition to the numbers you posted is a) how many of those people stayed on the site longer than a couple of pages, becasue that's what you'd really have to compare; and b) how many of those clicked on any of the moneymaking thingies (as well as incoming PW revenue and stuff), because that's what you'd have to compare with the sales. (Well, since your focus was on the exposure rather than the sales end, maybe it's a bit unfair to ask about that. Never mind. ;D)
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