my players are working for a hutt that likes exoctic creatures.. so they bought some.

they want to sell them to him but I don't know what I'm gonna pay them.

How much are exotic animals worth? They bought a Duracrete Slug and a Dinko

Price is all about rarity and value. The harder it is to find, capture, and transport, the higher the price. IF it's hard to keep alive though, it might not be worth as much. Also special circumstances can help modify the price (A Panda is worth a certain amount, but if I have a breeding age male, a healthy breeding age female will be something I'd be more willing to pay a little extra for)

So a Duracrete slug wouldn't be worth a whole lot, unless it was a really big one. They are more pest then prize.

And Dinkos also wouldn't be worth a whole lot, they are exotic, but not good pets and are already available in stores.

Might I suggest "Beyond the Rim"? There's info there on Nexu. Harder to get, more useful and valuable...

Other options would be Rancors, Aklay, Reek (not super rare, but interesting and not too hard to care for), Ghest, Stohl, Dragaax (with sharp pointy teeth!), Oskan Blood-eater (fun for the whole party!) and well you get the idea...