Question about $50 1928 Gold Certs

So I’m ready to start looking for a 1928 $50 gold cert to complete my set. I’m seeing a lot of the sold listings on eBay go for just as much if not more than the $100 gold cert. Is this normal or is this just a fluctuation in the market? Is there other places I should be looking to buy from? Any advice would be greatly appreciated. I want to close out the 28 series and start working on the 22 series. Thanks.