How are they asking and what are they asking for? You're providing a business service, it should be pretty obvious you'll be charging them, unless you haven't in the past and they assume it's a favour.
You could ask them what their expected budget is? I put this on my brief that I ask them to complete and explain it's so I have an idea if I'm going to be able to deliver what they're after in the time their budget will allow...
Asking for their budget/preferred price range is a good starting point. But really, the conversation should START with money, or at least business, at the fore. You can also ask if they are more comfortable with hourly or "stipend" style pricing when they ask you for work/mockups/whatever.
Don't work for free. We're professionals and people need to understand that. We're paid for our time.
Find out what problems they have and ask what having that problem is costing them...EG We keep missing our press deadline for our magazine ad...It is costing them time and stress are the full page ads are expensive... You can then show them how using you at a fare (not cheap) rate can solve this problem.