My exerience with high tech mains matches Steve Thruston's input. I bought my first one in 1998. It was a kevlar main with kevlar tapes (UK). In 2004, my second was a kevlar main with carbon tapes; again (UK)- we won our club season overall with this sail in 2008. I bought my third one which we hoisted in 2009. It is a Fusion X by Quantum. While the carbon taped sail was at the high end of price at the time, the Quantum Fusion X is not the most expensive sail from Quantum and probalby is less than the 25% premium over dacron.
Regarding performance, I admit to the placibo effect. Just kidding, but I do say this to my crew - "the bottom is spray painted and sanded smooth with 2000 grit sandpaper, the sails are hi-tech and in great shape. Gentlemen- there are no excuses, performace is now dependent on sailing skills - sail like champions". Too, my competitors believe I have an edge with my bottom and my sails and they are thus already beaten when they leave the dock!
Back to Mr. Thurston's input; he seems to support my arguement on cost. If you are buying a sail for racing, even at a 25% premium, the useful life of the high tech sail (5 years) more than offsets the the lower cost of the Dacron sail at (3 years useful life). My simple math says; with 25% premium, 4 years useful life on the high tech sail is a breakeven compared to dacron.