The Rich Text Editor in Sitecore is equipped with a lot of buttons, but not with a button to insert iframes in the page. Some clients, however, use iframes in their websites, but it is too risky to let them edit the HTML itself. So what we need is a custom button in the Sitecore Rich Text Editor.
For most content, Sitecore has a dedicated Web Control that renders it: sc:Image, sc:Text, sc:Link, etc. But in many cases, something like sc:Currency would also be very convenient, for the rendering of prices. Sitecore has not yet provided a solution.
The solution at http://deceptacle.com/2011/07/07/sitecore-money-field/ is not sufficient, because it does not have Page Editor support: the output is not editable in the Page Editor. So we have to modify it a bit.