Figure a budget

We are a small porivtae school with 50 systems and a server (server2003)
When I arrived 6 months ago the sevre was XP with a P3 processor and 512ram and a total of 13 computers

Through scrounging and begging I now have 50 computers all XP.

How does one start to estimate an annual operating budget for upgrades and replacements. Is there a magic number?

I have asked for more$ (They spent a total of 2500 this year!) By the way, I am a full time volunteer.

Thank You