They are about a 1 in 500 drop, if I recall correctly from one of the data collecting websites.
A guildie of mine got one recently and after he put it up for 9k buyout, I strongly advised him to try putting it for at least 12k. He put it at 12k and it sold within 5 minutes. Since then, I have seen a few for sale around the 15k range.
This pet actually happens to be truly rare and worth it's value. Pets such as the Aqua Strider, Grinder, Pandaren Spirits, etc all have their initial appeal driving prices up at first, but as their markets become saturated, they lose value fast. Same thing happened with the Raiding With Leashes pets. Porcupette will continue to be expensive due to the low drop rate from the pet bags (except the Pandaren Spirit bags, as they only drop the Spirit pets).
I was lucky and got a Porcupette perhaps only a month into trying the pet battle system. I have easily done 100+ dailies since then and have not seen another.