Buying antique piggy banks is tricky if you are not an expert. Most antique earthenware can be identified by a factory hallmark, piggy banks also!
If you want a particular antique piggy buy it for the price you have in mind and don’t forget to negotiate with the seller.
Don’t expect that the antique piggy bank becomes more valuable getting older, like old paintings.
In the case of piggy banks that’s utter nonsense.