A very cost effective aero wheelset that I have owned is the Rolf Vector Pro. Pretty much the equivalent of Ksyriums, a bit more aero, and typically they sell for $200-300 a set on ebay. If a spoke breaks, you have a problem due to the very low spoke count, but I rode mine a lot and never had a problem.
The only downside to the very stiff aero wheels is a rough ride. My Rolfs gave me numb hands. Eventually replaced them with a set of used Zipp 404's in tubie, that definitely cured the numb hands problem. The CF rim does an admirable job of damping road vibration. It's not uncommon to find used 404's on ebay for $800.