I will be spending a weekend in June next year (Thursday to Sunday) in Birmingham. I will most likely sleep in some cheap place in the city but I will spend days at National Exhibition Centre which is close to the airport. Because of this I will commute using train between New Street and Airport railway stations every day (probably from airport to city on Thursday and then city -> airport in the morning, airport -> city in the evening on following 3 days). 

What is the most cost-effective way of doing this?