I have a Kill-A-Watt meter... my iMac takes on another 35 watts as soon as I click "start generating" on the website.

In Safari it says it gets about 2500 khash, a share every 0.48 hours, which is impressive on its own for something running in a browser.

So a share measuring the CPU uptick only, is 0.035 kW * 0.48h = 0.017 kWh

If a share is worth 0.0024 cents and the 2500 khash is sustainable, its payout is similar to the power cost, so it is "profitable" to the user only if they are managing to stick somebody else with the costs.  (Something it shares in common with common crime)

Notably, this rate is higher than what others are reporting, it could be that Java for the Mac runs better (it's an i5 3.1GHz quad core processor).  Running this in IE / Safari for PC (in a virtual machine on the same Mac) consumes the same power but yields far less output.