What @Trakhak stated + read the Totally Tubular - Bike Forums
I would add that if you get the low cost tubulars, be careful about how you stretch them. Don't treat them like a rubber band but try to distribute the stretch around the tire. The reason is a possibility of inducing a bulge in the tire. I think it has to do with tread count. The lower thread count, the less costly. My preference is 320 thread count.