I can find information that says there are three upgrade prices
$15.00 if on a listed system between June 2012 and Jan 2013
$40.00 for upgrade for some systems - can't find the details of which ones qualify
$70.00 for upgrade for some systems - can't find the details of which ones qualify
All the above are download files only, not DVD media, and it's to Win 8 Pro, I think. Also, you should check the various versions and what's in them before you buy.
That doesn't include any hardware or other software that may need replacing.