Jesus I can see a lot of people are split on the pricing issue.
You can't have free in and cheap pints of strong beer or there won't be enough sawdust in the country to mop the floor of vomit and celtic jerseys.
That said, if this is a promotional event, there needs to be incentives for those that are genuinely interested so how's about €10 in and that gives you a card with the name of each brewer on it. The card then gets you a sample off each brewer free of charge, just a small glass, and the brewer then punches a hole across his name. When you're card is completely punched, put your contact details on the back of the card and place in the hat for the prize of a free keg rental with a keg donated from each brewery so there'll be a good few winners/prizes.
This has many advantages in that you make the draw at a late enough hour and only those present can claim their prize... that keeps people at the event buying more beer and keeps the numbers up throughout the day which ensures a good buzz about the place.
Also, it goes without saying that you utilise all the info collected on the back of those cards for market research purposes.
Finally, you hire hot promo girls (fact of life) dressed in the bavarian wench style going around giving out random prizes of little bits of merchandise like branded keyrings, (should have branded pens for people to fill in the contact info on cards), tshirts, tankards, etc. These prizes can be allocated raffle style with a raffle ticket given away with each pint. Only come up with something less Fr Teddish than a raffle.