use the PC parts picker service... it will check all retailers and find you the best price. just google it and check it out.
Windows 7 was dirt cheap 2 years ago..., under $50 a copy when they wer shoving windows 8 down everyone's throat. The public spoke, said we dont like windows 8... so they jacked win7 prices back up.
depends what you want it for on what your best option is. if you're using a machine still based on XP... you're probably better off to buy a $300 notebook with win 7, as it will be likely 10x faster tahn what you have.