OK, Sunk costs are cost already incurred in the project, which shouldnt afect future decisions.

But how about this?

"You contract an independent firm to audit your project. The payment agred is 110.000$ (30.000$ at the start, and 80.000$ when the study ends). The study is halfway done. What is the sunk cost?".

Should be 30.000 because that's what have been paid already?

Should be 110.000 because that's the total obligation acquiered?