Here you go:
Code:
<html>
<frameset rows="0,*" border="0" frameborder="0">
<frame name="_blank" src="about:blank">
<frame name="http://your-paysite-url.com" name="_content">
</frameset>
</html>
That oughta do the trick for the first bit; getting the paysite join button href to open a new window (or at least switch to showing the paysite url) isn't that easy, unless you want to use some javascript and rampage through the DOM tree modifying all the A elements
