Staples has been offering $2 for each used inkjet ink cartridge returned for recycling (up to 10 per calendar month for Base/Plus customers or 20 for Premier customers). This can result in $20 per month of in-store credit (that expires in 60 days).

As of 15 March 2013, their policy is changing and they are only providing the recycling incentive to Staples customers who regularly purchase ink from Staples (at least $30 on ink or toner in the past 180 days).

The specifics of the new program are detailed below. [source] You can click either image for a larger view.