As much as it may be a favor, your customer would spend good money to take these same customers out to lunch at a restaurant. Also your chance to be in front of potential "new" customers.
Come up with a menu and price accordingly based on your costs. I would suggest something a little upscale - Possibly plated and served (You'll need a couple of extra hands). Steak is just a little tricky do to because of preferences - Med rare, etc.
Would also talk with your customer to understand what/how they are trying to impress their guests - Is it a "thank you" lunch, is it a sales lunch, or a VIP event? Sell the value you bring to them by understanding their business and what they're trying to accomplish at this event with their customers. That's a bigger favor than a lesser price (to me).