If I am the interviewing in this case, do I tell the candidate all the ideas to lower prices/reduce costs?

Absolutely not, the candidate has to propose his own ideas. This is the way they test you "business sense" during the interview.

Best,
Luca

IMO you shouldn't; instead if they are completely stuck maybe you can suggest that they brainstorm and see how they approach the task.

Hello!

You should not, even when they get blocked, since it´s precisely what you are testing in the case as an interviewer! In the event of the candidate getting stuck and dry, you can suggest some hints for him/her getting back in track. 

Hope it helps! 

Cheers, 

Clara