Gift card appraisals

I've been thinking about opening a gift card shop for a while now and selling the gift card codes for less than their value.

I'm going to list the cards below, please give me an appraisal on how much I should sell each one for:
Steam $100 Gift Card -
Steam $50 Gift Card -
Steam $20 Gift Card -
GameStop $100 Gift Card -
GameStop $50 Gift Card -
Xbox $50 Gift Card -
PlayStation Store $50 Gift Card -

I'd say half off or 25% off would be more than fair