'Cheap Beer' Turnstile Energy

Put two beer stores a hundred metres apart but separate them with a plurality of turnstiles which generate energy when customers push through them. On Store "A" put a large sign guaranteeing the Cheapest Prices on Beer in town. Store "B" similarly guarantees to undercut any competitor's beer prices. (Limit 1 dozen per customer per day)

Customer goes to Store A and gets a written price on beer which he must then take to Store B who then slightly undercuts Store A in writing. Customer then goes back to Store A with B's written price where the guarantee is honoured by another price cut. Thinking he is onto a good thing, the customer goes back and forth incessantly until the beer is practically free or so cheap he can't be bothered any more.

The customer goes away happy with his ' shrewd bargaining prowess' unaware that the two stores have generated more dollars' worth of energy from their customer-powered turnstiles than the beer was ever worth in the first place. What's more, the customer has worked up a raging thirst so goes through his beer in short order only to come back again the next day to restock. Everyone wins.