Regular price for photoshop cs6 on its own is around £660. The student and teacher version is around £190. I doubt that new and legal version of the software can be bought much cheaper than that, which leaves either:
A 2nd hand version of possibly an older version.
If you only need it for a short time then use the free months trial version of CS6.
An illegal download version from somewhere for free. I doubt it would be hard to find.
A copy of the CS2 version which can be downloaded for free from Adobe (although legally you shouldn't be using it unless you've previously purchased CS2)