"Do you think I'd be working here, if I could afford a real snake?"
-Some stripper in Blade Runner.
I'd make exotic critters very difficult to buy. The best solution is to make the player(s) do a run for it. That way, the critter will be more valued and you don't find the player trying to buy lots of them later on. I see real pets being rare in an age where everyone just gets a more convenient robodog or whatever.
A baseline cost could be figured by comparing to drones of equivalent power, assuming that the critter will be filling a similar role in the team. That might not be realistic, but it will be balanced. And as GM, you can justify whatever price you like, really. A critter will be both better and worse than a drone. I wouldn't block a player from having one though if the player will really get a kick out of it. Be sure to stick in a few wide perimeter walls for prowling across and skylights suitable for leaping through so the player can enjoy seeing his great cat landing on security guards.