This is a great question! Before I can provide a solid answer, are you planning to purchase the phone from Sprint (ie a preowned phone) or from a friend/acquaintance? Let me know!
I can certainly make a guess. With devices like that, you should not have an activation fee charged to your account. I do want to warn you to get the device checked out before you purchase it. If you can get the DEC ESN from the person selling it, you can call in and have it checked to make sure it is not flagged as lost/stolen or anything negative like that.
Check out a fun infographic that teaches you how to increase storage, enhance security, improve battery life and clean your device.