what is a reasonable price for '040 on A2000?
Since they're not that common the price completely depends on the state of the highest bidder's brain. :-)
Apart from that, when you're referring to abovementioned Zeus card, I'd say around €100 - €125. Not more since it has some drawbacks like no 100% autoconfig-ed memory and some badly written drivers for the SCSI controller.
EDIT: as for other models: €125 to €150 is a price you could prepare to pay, depending on clock frequency, max amount of memory, SCSI on board etc.