This extension will let you add a specia price if a customer has already purchased a different product. Customer will need to loging to see the discounted price and not be available until after a successful order. 

Best explains how this works with an example: Our most popular extension is the advanced email template.
- Default price: £12.50
- Special price for upgrade, if customer has already purchased the default extension.
- Special price for repeat discount, if customer has already purchased the same product.