So I actually work in Tax so idk why I'm even arguing this (and I've never heard about that study but I'll take you word for it) but if we look at this from an expected value angle:

Take an example of a $100k audit that has a 15% (I'm taking the high end of the figure you gave because I think quality of audit firm matters as well. More reputable audit firms will be able to boast higher discovery rates) chance to find $1.5m in fraud (example).

The cost of the audit is $100,000.

The probability of finding $1.5 million in fraud is 15%.

The expected value of the audit can be calculated as follows:

Expected value = (Probability of finding fraud) x (Value of fraud found) - (Cost of audit)

Expected value = (0.15) x ($1.5 million) - ($100,000)

Expected value = $125,000

Therefore, based on the given information, the expected value of the audit is $125,000.

This means that on average, the audit is expected to yield a positive return on investment of $125,000.

Sounds like +EV to me