Depending on the boat and use it might not be such a slam-dunk. I paid $14k for a high-tech main and genoa 6 years ago, including a Fall seasonal discount. I burn about 1.2GPH at cruising speed about 80% throttle (7.5-8knots), about 1GPH if I cut it back a bit to 7 knots. So that's roughly 7nm/g and I could do even better if I cut it back another half knot or more.

As they say, YMMV. As a day sailor and vacation cruiser; if I cruised about 1000 miles a year under power (I don't), that's roughly $575 in fuel. (1000/7)*4 dollars per gallon. Over 10 years that's $5750. My $14k laminated high-tech sails have an 8-10 year expected life. Sails are expensive, even if I bought a trawler that gets half the mileage.

Clearly you could change the equation by using low-tech Dacron sails at half the price, and assume you could get 20 years out of them (not without re-stitching and other maintenance). You could also use different numbers for amount of motoring and the higher mileage of other boats (or lower speeds on my boat), but the main point I'm trying to make is that unless you're going long distances it's not clear which is cheaper.