Pretty low, it gets closer to 1/3 if you have almost all the Princesses available to be sold, so if you have only one I'd say a little over 1/50 :S
Oh boy I hope my RNG is good in that case.
Are you planning to change that in the future like when the Knight has high Affinity with the Goblins and/or Goblin Princess he gets acess to auctions with higher chances for Princesses and rare slaves?
That's definitely an idea I'd consider.
