Thank you for your answer! Nice job, but there are some problems.
Here is the correct way to ask using your words;
Excuse me, is this the right bus for the Opera House?
How much is it? How should I pay?
`a` is for something that is not special or unique. `the` is for something that is special or specific.
The Opera House is a famous building, so we should say `the`. Most cities have one opera house only, so in most places you would still use `the`. If you happen to be someplace where there is more than one opera house, and you do not have a special opera house in mind, then you can say `an opera house`. In this case it must be `an` not `a` because opera house begins with `o`.
Your second sentence is OK, but not native English. If we asked this question using your way, most people would ask `What is the fare?` We do not need to repeat `to the Opera House`.
Your last sentence is also OK, but we do not need to ask this in most situations. If you need to ask you can say;
How should I pay?
How can I pay?
How do I pay?
Here are some more useful phrases:
Does this go to the Opera House?
Is this the bus to the Opera House?
Are you going to the Opera House?
You go to the Opera House right?
Can I get to the Opera House on this bus?
Does this stop at the Opera House?