On October 22 Microsoft will offer time-limited half-price preorders of Windows 7 . In the US, this half-price offer has expired.
Under this deal, a Home Premium upgrade was $50 USD, and a Professional upgrade was $100 USD. If you missed out on this deal, Microsoft has confirmed that it is offering a "Family Pack" for Home Premium in "select markets".
The Family Pack will offer 3 Home Premium upgrade licenses for around $150 USD.