The EU budget is relatively small, at €185 billion. UK's net contribution amounts to £8 billion, a bit, but not earth shattering.

This Tax won't apply to all members, just to the ones that opt-in (Austria, Belgium, France, Germany, Greece, Italy, Portugal, Slovakia, Slovenia and Spain seem to be interested). And from what I understand, any revenues steaming from the tax would go to national governments and not the EU budget.