Probikekit.com has an excellent deal running right now for Ultegra 6700 cassettes (as light as Dura Ace 7700 but using all steel cogs). They are cheaper than you'll find 105 10 speed cassettes in the US at <$64 and free shipping. The only downside is the lack of a 12-27 but the 11-28 will give you even more range and may negate the need for that compact crank.

http://www.probikekit.com/display.php?code=D0026