I would like to survey a community of prospects about a new service we are thinking of developing. A question I would really like answered is "How much would you be willing to pay?" The problem is people, well, lie, expecially when it comes to money. Frankly, in the hypothetical they may not even know what they are willing to pay. Yet I would still like to come up with a reliable stake in the ground when it comes to price. So what survey questions would result in truthful and accurate information?
Asking Them Directly
The key is to ask them when a) they already have in their mind a good of what your sample product should do and b) they are relaxed and conversational, and know for sure its not a sales call.
A great way to get them in this state is to start them off talking about their business process, what they like, what they don't like, and what their problems are. Then mention some details about the kind of thing you're working on, making sure its obvious its just something thats 'in the works' so they don't start to feel you're selling them something. Then if you think they understand the concept, ask them how much they'd pay for something like that.
For the reasons you mentioned, you won't get a straight answer. But the number should be around 1/2X-2X of the actually answer. This can be enough information to put you on the right track - e.g. sometimes people will say $200 for something that you were going to sell for $20.
Throwing a number out
People are bad at putting a specific number on how much something is worth to them, especially if the product category doesn't exist. In this case if you have a large enough audience to get a good sample, just throw a random number out and get their reaction. Sometimes they'll even give you a number in their reaction.
You: would you pay $200? Person 1: Holy crap no way. I wouldn't pay more than $40 (way too high)
You: would you pay $50? Person 2: Yea I guess. (about right)
You: would you pay $20? Person 3: Yea definitely, that would save me so much time. (maybe you can go higher)
Guesstimating through other questions
If you can't get them in the right mood such that you'd feel comfortable asking directly, ask questions that will give you a sense of what they spend on products like it and how much their problem is costing them.
If your new service is like other services then you could check what they are asking.
Easy: Ask the customers to pay what they think it is worth. That way they are putting their money where their mouth is.