As long as you are not using said 'value' for tax purposes, I'd set the amount as what you would list them for if made from purchased goods and you were selling them outright. To jack up the 'value' because it's an auction and you think the bidders want to feel they are getting a deal makes no sense.
Charity auction bidders pay little or no attention to the 'assigned' value of any item, high or low. They bid what the item is worth to them at that moment with a bit extra added in because "it's for charity". Don't try to psych them out.
Bottom line, use similar item retail value.