Depends on the gambling site. It's always worth checking a few to get the best ones.
For example, Duelbits tend to have higher rates for underdogs than any other sites. I've just checked and the odds for Barcelona Vs Bayern are as follows:
Barcelona: x2.05
Draw: x4.00
Bayern: x3.10

Surely it will be an interesting game. Lewandowski has been performing really well recently and I wonder if he'll manage to score against his former club.