We don't keep old housing / cables as we've had problems in the past with proper storage and also just generally being too rusted to use again.
We have a suggested donations list and currently have cables at $1.00/cable & housing at $0.50/ft. This is definitely undercutting shops but it hasn't been an issue for us thus far. Possibly due to us serving different clients than our local shops.
Attached is our Suggested Price List. I believe we started with the Suggested Donation list from the Bike Dump in Winnipeg (thanks!) and modified from there.