I'd say asking them as well, "What do you think you'd pay for it?" could be useful to establish if it's genuinely worth going after it, but it's a really awkward question to fit in and it's a really awkward question to answer as well.
I like to judge the awkwardness of their response. It helps you learn how serious they are. If they really struggle to find a price they probably don't think their problem is that painful.
You can also focus on how much a problem costs them in wasted time or lost revenue.
I think it's very difficult to estimate what something is worth to you unless you're actually on the spot for the buy/no-buy choice. As an example, I was looking for some good software to help me write Chinese characters. Before I went looking, I would have thought it was a $10 product, but the leading product is actually $10 a MONTH. Way more than I'd estimated, but there are good business reasons for this. When you're faced with that choice of buying it at $10 a month, it becomes more real. I didn't buy it yet, but it's a very popular product, so for many people it's obviously worth it.
A better question to ask would be "Imagine there is such a product/service that solves your issue, can you guess its price?" Let's say the answer comes out at $100. Follow up question may now be "Would you pay if this was available to you for $80?"
Not if you're honest about what you're having the conversation about. They know you are doing research for a startup idea. You can even generalize the question... "What do you think solving this problem is worth to businesses in your industry?"
