How much is this worth?

I own a small sign company in Michigan. Every year for the past 19 years i have made $10.000 profit from the same company for making thier signs for them. It is a two week project and is just replacing the old signs with new ones. These are speciality signs made from gl. I have a life time contract with this company that only my company can supply them with these signs. In other words I will make $10.000 profit for as long as I am able to supply this service or I can sell now for a set price. I have been contacted by a larger company that would like to be able to supply this company the signs and take over my contract. They would be willing to pay me for this. My question is how much do I ask? I am 59 years old and I am slowing down a bit and am eliminating some of the companys I supply. That is why I am selling this. What do you think this deal would be worth? Thank you.