The $4800-$4900 prices are from 2016 before Stern increased their prices in 2017.
$3900 is a good price. Anything below $4500 for a Stern Pro is a decent deal. This example could've been plagued with the Stern quality issues commonly seen on the early run Ghostbusters.... or maybe not. If it saw some heavy commercial use, that would've dropped the value, too.
I don't think the IFPA pricing is really relevent for comparison considering most people can't get it.