I think it was Beakybird (Larry) who worked out what it cost him to upgrade from two PSR-S900 to two PSR-S910 over the period of time he used the former.
It was only a relatively small amount of money per day.
The Big Three, Yamaha, Korg and Roland, hold their value very well...Casio not so good, but still a great alternative, as their initial price is generally lower.
Ian
_________________________
Yamaha Tyros4, Yamaha MS-60S Powered Monitors(2), Yamaha CS-01, Yamaha TQ-5, Yamaha PSR-S775.