Couldn't say, how much exactly is beer in Prague (where prices are usually higher), but here, in south Bohemia 1/2 litre of Pilsener Urquell (which is probably the best/known from Czech beers) costs cca 16-20 crowns (ie. 70 - 80 US cents or 50 - 60 euro cents).
ADD: About t-shirt prices I couldn't help :-(