I haven't really seen a "market" price for breeding pairs, because #1, everyone wants them, and #2, there isn't a huge market for pricing. Meaning not a LOT of people sell breeding clowns.
However, I have found 3 pairs of breeding clowns, NOT Clarkii's though. Normal Occelaris - and IMO, below "market" price. They are priced about $30 more than a normal "mated" pair. A good value if you just wanted to breed them for fun.
Usually, as I've seen it, it breaks down in pricing like this:
1) single
2) pair
3) mated pair
4) breeding pair