You wouldn't be happy using 10-speed brifters with a 9-speed drivetrain.

I've been getting around 2000-2500 miles out of SRAM 10-speed chains, and I ride in a lot of rain. Now that Tiagra has gone to 10-speed, there's a relatively cheap source of cassettes, but I have yet to wear out a cassette (though my tolerance for cassette wear seems to be significantly higher than average -- as long as everything shifts fine I keep my cassette).