If you have a pure electric vehicle you're a captive audience and have no choice. That's who the pay charging stations are really aimed at.
With the Karma (or the Volt) the range extender gives you choices, and your math is correct. On a strictly dollar basis, paying $2 an hour to charge is very expensive, and not at all worth it. For me, it's also about 500% more than I pay charging at home. There were a few threads about this many months ago, but the general consensus was Free was a no brainer, $1/hour was about as high as most people would go, and anything more more was ridiculous and not worth it. For me, I'll use free chargers where I can find them (they are scarce, but growing in Atlanta) but I've never paid to charge.
Brent