Don't delete them: segment them. Move them to another list for their own retention campaign. Send them a series of 3 messages saying that you've noticed that they haven't read anything from you lately, and you want to make sure that they're still interested in receiving information from you. If after that, you still haven't heard from them, delete them. In addition to keeping your costs under control, it will also improve your open/click rate with your sender, which, depending on which service you use, may be important, as it's often taken into consideration if you get a lot of unsubscribe requests or spam reports.