Peanutty is correct, prices vary drastically, like him/her I have seen the average on the ji-kun be about 10k. Raiding with leashes varies more in my experience though - I've seen them from a couple hundred gold (for the MC pets) to as high as 6-7k for pets like Anubisath Idol - they're still rather all over the place. Kind of depends on the amount of a pain in the hiney they are to farm solo lol.
A good idea might be for them to check the trading forums here to find the pets they're after, there's often people with spares that are willing to trade with other people for something different. And although people can value each pet differently, they're generally similar and often willing to work something out, I know of a few people that'll go hunting for a pet you're after if the trade is acceptable to them
. Good luck to your guildie!