Ai: That refers to the September cake dogs, the Nick cake dog was released later. That said, isn't the Nick Cake dog worth a common September? I can't double check because someone deleted one of the resources I use for references, hopefully it'll be recovered soon.
Centri: The Easter dogs aren't worth anything in the trade because the other pets are so much more valuable, but the advent rat, soxic/sortic and gryphon are good. I'd say it might be fair, but it's definitely not overpay; Joker is very hard to get a hold of.
Smoothiemaster772: Technically unfair to Gearback, but people pay outrageously for Jokers sometimes, so it's your call. Gearback is also fairly popular I think. Maybe get an opinion from someone that's traded Jokers before.