Suggestion: "how much would you theoretically pay for this" won't give you much accurate information. I'd suggest changing it to "pre-register for one of our plans: free, $5, $25", with an overview of each, and see what people click.
When you put such an option to people, they will not tell you how much it's worth to them. They will only tell you how much they are willing to pay... if at all they had to. Expect the numbers to skew heavily towards free.
Actually, I'd say, once you get the data you should generously double the price.
Example here: http://www.quirky.com/products/327-The-Milkmaid-smart-milk-j... (though that product might be out of price survey mode by the time you click through)