I am implementing an back-office application where the user type in prices for products. Sometimes it is preferred to type the price inclusive value-added-tax, VAT and sometimes exclusive VAT.

How do I best let the user choose between the inclusive or exclusive VAT in a usability perspective?

I could have two TextFields above eachother one inclusive VAT and one exclusive, and reflect the input. But I don't think that reflecting the input in another TextField at realtime is good when it comes to usability, it distracts the user.

I could also have two Radiobuttons above the TextField or below, or maybe besides the TextField that let the user make the choice.

Or should I have a single button for turning between inclusive/exclusive VAT? like On/Off-buttons. But what text should I have on the button and how should the button be designed? I think this is good because it takes less space and it is easy to have it close to the TextField, but it's very hard to design a good button in a usability perspective.

Please give me some recommendations. Maybe someone of you works with usability or have seen a similar problem.