Yes, one can underestimate the money required and more particularly the time it all takes
And time is money.
The maintenance of a Multi-engined IR
is not cheap for a Flight instructor !
'.. nice chap, plenty of experience but needs a bit more multi-engine time ...'
' ...no jet time .. no multicrew time
..MCC?... LOFT?... Type rating would be nice
Time on type Oh dear.....' etc etc