Put a number on it, otherwise you're just blowing smoke. If you can't put a number on it, then you clearly haven't performed the analysis and so your claim of it not being a good use of money is untested.
The same way you actually fix the problems. By figuring out the steps you need to take, and estimating their cost. If you can't do that then how are you going to fix the problems even when you do have the money?
Having worked on anti-abuse/fraud work, I think $500k is light for a site the size of wikipedia. It takes people working full time to create and modify the systems to stop it.