Given that Boeing provided this substantial funding a while back, part of the process would surely have been to do their Due Diligence on the business, going through month-by-month the forward projections for cash flow for all the costs and revenues, and seeing there was an adequate margin to ensure the deal. For all that we take the mickey out of beancounters in the industry, this is where they earn their corn. If Boeing, of all people, are not able to do this properly then the industry is in a sorry state. That would include the handback costs for the Airbus aircraft, which will vary by lease and by individual aircraft (let's call this 'dilapidation costs') as well as everything else. It is most definitely not a 'back of a fag packet' calculation.
The existing lessors probably had their own provisions for this, and of course they were stuck with the leases. For Boeing however it was all a commercial go/no go situation. If there is a formal enquiry into the failure it would be good to ask Chicago some penetrating questions.