There are a couple of sales landing pages for real products that I could suggest looking at but none that specifically ask "Would you buy this if we made it?" They can give you an idea as to how to at least format a page to achieve the results you want.
- Copyblogger has 3 software offerings that are explained on the landing page. One, Premise, is even focused on landing page optimization
- The Art of Non-Conformity's Unconventional Guides has a series of products that show a great way to layout a sales page. When you finish a guide, Chris sends you to a survey to ask what you might want to see next.
- Lateral Action's Money for Creatives has a similar feel to the Unconventional Guides but with more depth and detail.
What you are really doing is something called A/B testing (which you can read up on here) to figure out which features, functions and price points will get customer to convert.
Usually, A/B testing is done with a real product and real pages. It will be really hard to gage what a customer will want without actually having something to put in front of them.
Most won't really give you a "Yeah, I would buy that if you made it." Rather, it's more like "I bought it but would like to see these features."
I would suggest scoping your idea via a Minimal Viable Product (MVP) and go build it and then do your testing. I know this is more time, effort and money but the results will be a lot more meaningful.