get iFrame's source's variable

Hello!!! If someone could help me I'd really appreciate it..

I  have a web page made with EXTJS with an iFrame.

The iFrame's source is an aspx page and has a variable like this: <var id="myVar" runat="server">var>

I need to access this variable from my page, wich, since it's EXTJS, is in javascript...

I guess the question here would be: How do I access an iFrame's source's variable from javascript??

thanks!!!