Like enigma said, this is a very shard-specific type of question, and also I tend to put the pup's resists and stats at a higher priority than anything else. After resists/stats, the rarity of the color plays a role, but a 4-star plain cu sidhe will command a premium price, too, if his resists and stats are stellar.
Examples of pets I've sold on Atlantic -
Red, white, black (the rare colors) - 350k, untrained, regardless of resists/stats. 1-3 mil untrained with excellent resists/stats (3.8/5 or higher on the pet power calc). Ice blue and pink are also rare, but they don't seem to sell as readily as red, white, and black, in my experience.
The highest I've sold a pup was a fully trained (5x GM) plain cu with excellent resists (-6 off max resists) and max HP - I got 5 mil for him.
I've seen partially trained rares (3x or 4x GM, red, white, black, ice blue, rose pink) go for 3 mil at auction.
If you're not sure of the rarity or power ratings -
Vet Master has color rarity:
http://vm.or.tp/ve/791/a3cusi.htm (scroll to the bottom)
Power Calc:
http://www.uocraft.com/taming/pet-power-calculator/
