You don't say what sort of business you're in, or what sort of clients you have, so it's impossible to say. However, if someone insists on a meeting and doesn't tell you what it's about, I think it's reasonable to discuss some sort of payment for your time and input.

You can make it clear that if the meeting turns into something that is of mutual benefit then it will refunded, rescinded, or otherwise paid back. But equally, make it clear that your time is of value to you.