In most pubs and bars, the price of drinks are fixed. Now if the price changed based on supply and demand, it would be similar to a stock market. However, in the case of supply, there would be only one seller with a virtual monopoly.
Hence price variation would only be based on demand.
A bar in Berlin actually operates such a beer market (or Brokers Bierbörse), the Berliner Republik. They have 18 draught beers with changing prices based on demand, after 6pm daily.
photo by Melanie from holidaycheck.com
Another possible variation would be to set prices based on public auction by customers with whoever bid the most purchasing the beer. Of course, it would depend on the customers. While most would prefer to pay the lowest price, others who could afford it might turn an establishment exclusive based on wealth.