I'm not sure about this, but I would think that unless you need something offered here
http://www.fckeditor.net/license/commercial/ then I think you would be fine using FCKeditor under GPL (by using the SJSD extension most likely). Of course you could just use the GPL official Online Editor :)
So this means if i am using the extension with the professional license software, then i have to pay another fckeditor commerical license fee which is around US $750 quite lot money.
Like most extensions (if not all). The professional license is only valid for the core product from ez systems. You should contact all extension authors of which you want to use their extension ... I think it is even more complicated: the extension authors would need to obtain a commercial license for ez publish themselves to sell their extension?