Great topic.
I went far back as to 2010. A PSA 7 Mantle went for $10,575 on the 1st of May. A PSA 7 Mays went for $4,112 on the 1st of December. It looks like the ratio for the PSA 7 was around 2.5 to 3x back then, so not much has changed.
And yes, I think the Mays is underpriced as well. I have heard all the arguments before, but to me the biggest one is the fact that he only won the World Series once. Mantle went to the World Series twelve times and won seven championships.
That's crazy!