JotForm is a free online form builder which helps you create online forms without writing a single line of code. No sign-up required.
We believe that if one user has a question, there could be more users who may have the same question. This is why many of our support forum threads are public and available to be searched and viewed. If you’d like help immediately, feel free to search for a similar question, or submit your question or concern.
I believe the Slider With Calculated Result Widget is suitable for your requirement. This widget allows you to create a slider with complicated calculations. To add the widget, simply follow this guide: How to Add a Widget to your Form?
Here's a demo so you can see it in action: http://form.jotformpro.com/form/41042444921951.
If you need further assistance, please let us know.