I have seen that the prices are dropping some but the ones that are real cheap are in pretty bad shape too. The cost of repairs is showing up on these old machines. When the combine is only worth maybe $5000 and the parts to put it back in good shape are $5000 them what do you do????
IF you can do the work and know the rest is in good shape then you may do it. If you can't do the work then your in trouble.
As for trading up from your JD 6620 to a JD 9400/9500. Make sure and get a later model of both. These 9000 series combines where a completely different design that the older 00/20 combines. They had some bugs that needed to be worked out of them.
One of the big ones in on the JD 9400. DO NOT buy one with the 359 motor in it. The 359 is great motor but they hung the auger unload drive out about 10 inches from the crankshaft dampener pulley. I have personally seen FIVE of them break the crankshafts right behind the dampener. It usually happened right around the 2000-2500 hour mark.
I would hold out and try to get into a 9410 or 9510. They had a lot of updates that are worth the extra money to me. Just the setting of the chaffer and concave are big ones. Then back to the 13 bar concave( Titan II days) and a digital readout on the concave settings.
Truthfully I think we will see lower prices on used equipment for the next 18-24 months. The swing in grain prices will effect the net incomes greatly. There is going to be major loses on many acres of grain in the next few years. HIGH inputs/rents will take time to adjust to the lower gross income.