You are going to have to try to determine the Fair Market Value.
I would start with the place you bought them at, and ask what they are worth or what they would pay for them.
Technology is rapidly changing in the area, so unfortunately they value will be significantly less than the amount paid.
Go to a number of hear aid places and ask, then try to get it in writing what the value is.
Also check the internet to see if you can buy used hearing aids and see what they are going for.
