Almost none, unless it was something I'd read every day + found more than 1/2 articles relevant + useful. Won't even pay for NYT if only 5 or 6 articles/month add significant value.
We need a subscription business model that aggregates content like cable/pay tv, so I can pay a fair price once, and continue grazing across most sites without worrying about individual subscriptions or micropayments. WSJ and Economist are only sources I would pay for (at a reasonable price) that aren't providing specific data points I can use. And, most people wouldn't find those as useful as I do, so it's down to personal preferences and how much of any source you'll get value from.
It's why individual subscriptions to everything is a dead-in-the-water business model that's going to kill off most journals. Most sites will be abandoned if there is a paywall.
Think about it this way -- how many people would pay for The History Channel or HGTV as individual add-on tv channels? Yet a very large number of people will occasionally watch those sources if it's included in general subscription, and they add value to the overall package, even if you rarely watch them. That's the way I view 99% of online content.
And, you have to make it dead easy, and not be constantly harrassing me about subscriptions or micropayments, or I'm gone. And, I also expect sites I'm paying for to be ad-free and not slowing down my browser, or I won't pay.