Javascript calculator for websites

Hi My client sells finance to dental practices for patients who require expensive cosmetic treatment such as implants. He needs coding for a calculator which can be embedded in a web page so that patients can see what their repayments might be over various terms. He will want to distribute this code to his client practices for them to embed on their own websites, so the code would have to be cross platform and cross browser. Different practices may have slightly different terms and interest rates etc, so ideally it would be good if it was simple for my client to alter the values himself if necessary. Neither he nor I have any working knowledge of javascript. He has put together an example calculator in Excel, which I have attached, and the final version of this is what you would have to work with. Although I am familiar with Excel, calculations such as this are out of my comfort zone, so you might have to liaise directly with my client at some point. We would need the calculator branded to match his website www.financingirst.co.uk. Please let me know if you are interested, and could you give me an idea of costs? Many thanks Ian