I feel like most of the people willing to pay 50-100 million bells have gotten Marshal, and the others either do not want to pay for him, or do not have the money to pay 75 million bells.
Times generally do have a part...I noticed a lot of popular villagers going a lot cheaper than normal
around Christmas time...I assume because they were quick auctions and most people
were afk with holiday plans.
As for Marshal...the highest I've seen him go lately is 20-30 million so I don't think he's losing
too much popularity yet.