How many hours will it take to add woo commerce to an existing website? Not a ton of products.
What price can one charge?
Still a lot of variables to consider 
But you can look at the prices for WC vs regular site on the pricing chart, and compare the percentages. Eg, let’s say it has an e-commerce site as 50% more than a regular one, you could charge 50% of your ordinary price for the Woocommerce addon.