500 (pounds/euros?) isn't much. You can't even really buy a tape drive.
Ok, first off. I am assuming there is zero backup infrastructure. Is this true?
Secondly, how much data are we talking about?
What platforms are you running?
How many clients do you need to back up?
How often does data change?
If you had a more open budget I'd just tell you to buy a tape drive, some tapes and a backup scheduler and run a job every night and take the tapes home. Since your budget is so tight I need to know some more details so I can recommend some thing that will work within your budget.