Spiller is much better than you give him credit for. He was, for a time, excellent. Nearly unstoppable. Obviously, that isn't sustainable for him and he does have problems blocking and making something out of nothing. But if there is space and you can get him rolling and he's healthy, he's very dangerous. Not to mention, his full potential in the passing game has yet to be realized but he is capable of putting up numbers there too.
If you can keep Spiller you do. He's a fantastic talent who you draft high. You don't need to overpay but you can't just let him go without a competitive offer.