Tim, the price needs to also take into account quality.  Age, breeding/pedigree, source reputation. I don't think you can say that a pair is worth XX without some knowledge of the pair. Some may be worth just pound price + 1/3, some may be worth more. 

I would figure the pound price + 1/3 would be the bare minimum they would sell for to justify the extra cost/labor, vet shots, preg check etc. of  the seller even bringing them to the pair sale vs the weigh-up sale.

If they were particular good for some reason such as pedigree or source reputation, then the price would go up from there. How far depends on how bad everyone else there wants them.

Jim