Again, take Questionable Content as an example. In the webcomic community, Jeph's comic is quite possibly the most popular out there.
Questionable Content is very popular, but it's hardly the most popular webcomic. For one, it's popularity is nowhere near infamous PA and Pvp, and then ther's a number of comics like Shlock Mercenary, Scary Go Round or VGCats, with measurable popularity.
Of course, it's far from what we have, but it's not nearly the most popular.
Compare him to webcomics, and he tops the chart in popularity. Compare him to YouTube, and the popularity pales in comparison. That's why popularity can't really be objective, because you have to take everything into account. Let's say the author for "Stick Figure Comics" is expecting absolutely NO readers, because, frankly, his comic sucks, and he knows it. But then he gets 18 fan-mails per week a year later. While next to nothing when compared to Questionable Content, I'll bet the author thinks that, all things considered, his comic got popular.
Well, if you compare two categories that have nothing in common whatsoever, of course you're gonna get wrong impression. I mean, you got to find an objective criterium. If you feel that your comic is suffering from once-a-week schedule, you find a well-known comic that updates once a week and compare to it. On the other hand, if you have a 7x a week comic and compare yourself to a popular 1 x a week comic, then you're just using the wrong measuring unit.
If you think that your comic suffers in popularity because of it's content, you just find a popular comic with similar content. God knows that all kinds of comics have been popular. From disturbing graphical sex like Sexy Losers, to family-friendly newspapers content like Kevin and Kell, to totally innocent children content like Ozzy and Milly, geek comics, gamer comics, SF comics, fantasy comics, historical comics, whatever kind you do, you can find popular webcomic that can be put into the same category and thus by seeing what your comic could be, you can conclude where it is now.
Of course, if you compare it to youtube, you get skewed results. Like a lab experiment that hasn't been thought through to exclude bias. But on the other hand, I could as well compare my numbers to the average of CG per comic, and decide that I'm incredibly popular, ommiting the fact that there's a couple of thousands of CG comics that have about 1 comic in the archive.
It's all about whether you want to be realistic or not.
Honestly? I don't know where all this merchandising and costs came into the conversation. I was referring to popular in terms of readers, money's a completely different can of worms that I don't feel inclined to pry open.
Number of visits is one way to measure popularity. It's marketability is another one. They are rather closely tied subjects.