I would like to say that the bigger the fish, the higher the prices... much higher! Since dats grow so slow, they command ridiculous prices when they are larger. For the regular "3 bar" ID, 18" is the max size, but it takes years to even get to that size. I've had mine for over a year now and he's gone from 2" to 6" and has slowed down considerably once hitting 6".
Which specific species are you looking for? There are 4 in the aquarium trade -
Datnioides (or
Cious. I'm not sure of the actual genus name right now)
microlepis (indonesian datniod),
D.
pulcher (Siamese Tiger datniod),
D.
quadrifasciatus (Silver datnoid), and
D.
campelli (New Guinea datnoid).
All can be distinguished fairly easily, except for some exceptions with the
pulcher and
microlepis (and even then, the price is an easy way to tell the difference... pulchers are $300+). One this is for certain, though: you won't ever find a pulcher in an LFS. They are mostly owned by private sellers or hobbiests.
Good luck with these fish! They are definitely one of my favorite fish. They are full of so much character and intelligence.