If you stick with Home Premium, the upgrade version is around $119. From Amazon, it's around $108.
If you want the full version of Home Premium, it will be more ($200 SMRP).
You could also get an OEM version which doesn't require a previous copy of Windows. Those you can get at Newegg for $105. Problem with OEM is you get only 32-bit or 64-bit, while the retail version (upgrade and full) give you both.