Undocumented, inherited, and too important to switch off. We take ownership of applications in that state: stabilise first, document as we go, then decide with you what deserves further investment.
Most firms will not quote this work, because there is no clean scope in an undocumented system. We price the first phase as an assessment precisely so the scope becomes real before anyone commits to the rest.
One week, fixed fee, written findings. Including the finding that you should not spend any more money on this.