The operative word is inexpensive. It shouldn't be too far off $20 and childsplay to assemble, and probably quite a bit of fun to tinker with as well.
Here's our I/O expander board, it's a "biggie"!
We're changing the layout to one VCC + GND "IN" and one VCC + GND "OUT" - and moving the CLD's IN and OUT top and bottom respectively, grouped and labeled "595in/out" + "4021in/out".
The board is based on these two excellent references:-
We know there are other ways of doing this, but we happen to already have tried and tested software which utilizes these low power inexpensive IC's. Turning it into a PCB, for us is a simple case of tidying up the otherwise messy wiring and prototype boards we've been using.
The boards will work with any of the Arduino's and we'll have a small surplus available for anyone else wanting them.