We can do some back-of-the-envelope math to estimate how much you could make doing this. Let's assume that the benefit from bitcoin mining is roughly equal to the cost of the electricity used. Electricity in the US costs about 12 cents per kWh. Let's suppose you can get your web site visitors to put 100W of extra power into mining bitcoins for you. And let's say your visitor spends 5 minutes (1/12 of an hour) on your page. Then each visitor is worth 12 (c/kWh) * (1/12) h * 100 W = 0.1 cents. Which would be like $1 CPM advertising.

But these are all best-case assumptions. Your visitors might not average 5 minutes per page. Your JS mining tool almost certainly won't be as efficient as the specialized GPU rigs that can mine bitcoin for less than the cost of electricity (and drive up the cost per unit for everyone else who's mining bitcoin). Or your users might not have an extra 100W of power to throw at your mining operation. Taking these factors into consideration, I'm guessing you'd be looking at CPM equivalents that you would never touch.

Also, I suspect it would be considered malware by many. Still, it's an interesting idea.