A possible solution: Create a New Question Type, use jqueryui slider
While the data structure for making this a single question would likely be the same, the UI could look like a range slider:

http://jqueryui.com/demos/slider/range.html

So instead of creating the question "How much would you be willing to pay for this product?" as a multi-numeric input with two subquestions "Min" and "Max" and having those show up as two fields, the UI could be represented like this.