Price Calculation Form in RS Froms Pro JOOMLA

I am creating a proof reading website. I want to have a form that creates a calculation based on the number of words inputted by the user visiting the website and the type of document. E.g. user enters 2000 words charged at £5 per 100 words for a piece of coursework. E.g. user enters 2000 words charged at £2.50 per 100 words for a dissertation. The cost then needs to link with PayPal to process the payment. This can be done in RS Form Pro with custom scripting. Please quote 'Proof reading Project' in your response.