In the list of rarities coins of Proculus are listed as common. At $70K that doesn't seem so common to me.
Proculus is only $70k because there are fewer of you who might consider buying it than one that we all believe really was a part of history. For example I'd rather have a Constantia issued by the real emperor from the real mint to commemorate a real person than bet on a uncertain usurper made sometime between 200 and 2000 AD. There are some people who believe that all EID MAR denarii are modern fakes starting perhaps 300 years ago. I don't agree with them but suspect that they would sell for more if everyone agreed that they were what we hope they are.
the auctioneer asked for a starting price of GB pounds 35,000 but failed to get a bid-this one found in Stamford bridge north Yorkshire nov 2012,the only other one in a museum in munich