When it comes to gun prices,a lot of factors come into play.
One is local availibility.A 5906 is a great gun,but you can't buy what isn't available for sale.As common as those types of guns are,some places-like my state of residence-there's just no way you'd ever see one come up for sale that often.I've NEVER seen more than one 3rd gen on sale in the same store since moving here.If I want a 3rd Gen before the end of the year gun show,I have to go online.
Conversely,I've seen 3 S&Ws on sale at a gunstore in Illinois once.They wanted $550 for a lightly used 5903 w/adjustible sights.They weren't there the next time I came to that shop 4 months later,which brings me to the topic of local prices.Some places have different laws than others.A 5903 which sells for 450 in your neck of the woods might go for $600 in a place like California or Mass. where a 3rd Gen can't be sold to a citizen outside of a face to face transaction due to roster restrictions.I've heard anecdotally that a used 5906 in CA can go for $550 and up because the only way to own one is a face to face sale by someone who moved in out of state.
Gunbroker can be a wonky site to judge prices because pf those factors and more.A guy in Georgia logs on and thinks $499 for a 5906 is nuts;he knows Joe can hook him up with one OTD for $300.Conversely a guy logging on from Lincolnshire Illinois thinks $499 is a bargain of the year,since the only 5906 he's seen in his area for the last 2 years went out the door for $600.
Personally,id say that the day of the collectable 3rd Gen is upon us when variants like the 5943 start creeping up in price.Right now only the well known 3rd Gens like the 1006,4506,and 5906 are pricing mid to high.