Windows 7 is priced from $200 (Home Premium) to $320 (Ultimate) for full retail version while the upgrade packages are priced at $120 (Home Premium) to $220 (Ultimate). There are three editions of Windows 7 to choose from for home user, the home premium, professional and ultimate. Other editions are not relevant for individual user, the starter edition is only available for Netbooks via OEM installs and the Enterprise edition which is similar to ultimate is for corporate buyers.
If you are a XP user you need a clean install to get Windows 7, if you are already using Windows Vista you can use the Windows Upgrade Advisor from Microsoft to check if your machine supports upgrade from Vista. If you are looking for an upgrade from windows XP to Windows 7 technet has some information on this.