How would one quantify who much CalPERS has lost to high frequency trading?

It is pretty much accepted now that high frequency traders are, through dubious relationships with various stock exchanges, front running the financial markets. The hardest hit are large institutional investors. While it might be hard for some of us to care about whether the Bank of Canada is having it's bottom line marginally effected, I think we can all be incensed that pension agencies like CalPERS are getting fleeced. This directly affects California taxpayers. This being the case, is there a ballpark estimate of the cost to CalPERS attributable to high frequency trading?