$7 per pound is average for Live Rock at LFS's. Depending on the type of clown you can see some high pricing of $2-$400 per pair easily. But if you go with pretty common types of clowns (i.e clarkiis) you can find them easily for $40-$50.
Best bet as far as clowns go. Buy a large 1 and a small 1...They are hermaphrodic. the large will turn to a female, the small will stay a male.
Even if they're paired or mated, it's no guarantee that they will spawn in your tank. And, it's very, very difficult to raise clowns from babies, even for veteran reefers. As soon as the fry hatches, they need to be removed form the tank and placed in a separate tank or they will immediately become fish food. Did I mention that they generally hatch in the middle of the night?
That being said, unless you want to attempt to raise a fry, there is really no (IMO) reason to try to find a mating pair of clowns. Just find 2 of different sizes. But the same type.