Wait...if you are going to sell the lens privately, you might be able to get @ $100 for it if it is one of the later VR versions. Used ones that sold on ebay run $69 to over $100 (for new). Less if it doesn't have VR. BUT...if you are going to trade it in at a camera store, you need to remember they will want to make a profit when selling the lens. If the current used price is around $100, they will surely offer you around half that so they can make some money.
You should go to B&H's web site to see if you can get a quote based on the condition of your lens. Some stores go by the price B&H offers for trade-ins.
Used Quotes
EDIT: Steve jumped in ahead of me while I was typing.
We are saying the same thing.