I'm sure there are other ways, but here's a possibility for those staying in the Bom Jesus hotels of how to get to the venue:
Find the elevator (ask at the hotel). It's powered by water.
It looks scary, but it really is... OK, scary.
Ticket down for one person costs 1 euro (ticket up costs 1 euro too).
The elevator runs from 08:00-22:00, every half an hour... or so they advertise, but I think they use the elevator whenever they have enough people too.
Once out of the elevator, there should be a bus stop in front of you, which has buses every half an hour too (I'm not sure, but I think the bus gets there a little bit before the elevator goes up and waits for people coming down before departing).
I don't know how much the ticket costs, but it shouldn't be much (in Lisbon it would be 1,20 euros, so in Braga it should be the same or less, I think).
Here's the map you'll see at the bus stop:
Just get off on one of the two stops pointed by fingers (thanks, Allison, thanks, Gloria) and you should be really close to the venue.