Not unreasonable in terms of the cost to the supplier, no. However to a 'user' (me) it is unreasonable as it costs as much as the Pi!
I think I have two ways to go:-
1 - Stay with my Beaglebone, it's behing the Pi in some ways but it does what I need for the moment. I'm just worried about its continuing future and development. 2 - Use a Pi and do the ADC with a couple of cheap, non-pi, ADC boards. It will then cost only a fiver or so.