If you're using them strictly for racing, I'd go with tubulars.

If you're going to do them on group rides, stick with clinchers.

I think the cheapest 50mm carbon clinchers (at a decent weight) under $1000 are Neuvation's http://www.neuvationcycling.com/wheels.html

For $927 you can get a set of 50mm carbon clinchers that weigh 1600g

Oh forgot about these

http://www.planet-x-usa.com/pPC50CL/...-Wheelset.aspx

$699 (not including shipping) for a set of 1740g 50mm clinchers.