How would you respond? What if the proof of concept takes labor and extra configuration to meet their requirements as to what constitutes a PoC? Should you charge for that?
In general, doers do and talkers talk. If you really need the service, you're going to buy it and figure out things yourself. Plus, paying customers always get support and help no matter what. If you start talking hypothetical scenarios and custom requirements, it's mostly bullshit you don't really care about. And it will never amount to anything.
In the words of Ray Liotta: "The place burned down? Fuck you, pay me. Lightning struck? Fuck you, pay me. Slow business? Fuck you, pay me." We work for money. Not to make your life easier out of the goodness of our hearts. Money talks and bullshit walks. Anything else and you're just wasting both of out times.
My suggestion is to ask for $10 pre-payment. Or have them sign up to your basic plan first. You will see a lot of prospects won't even reply to your email.
I've seen web development agencies even charge for meetings for Skype calls with prospective customers. I guess that's one way to filter out clients with small pockets.
I find it interesting that NONE of these people that have requested sample data ended up as customers. It was almost always a wasted effort. In fact, almost all the customers bought immediately and then worked with to figure out what needed to be done and in most cases quite okay with waiting.
I will have to agree that filtering might be good here.