Just to clarify the question of why Monarch themselves aren’t performing the rescue flights. In law, once a company is declared insolvent and ceases trading as they did at 4am they are no longer permitted to engage in any trade whatsoever. Undertaking a revenue flight, or receiving a fee from the CAA to operate rescue flights would be against the rules. Besides that, who’s going to fuel and handle a Monarch aircraft at the moment?
Just to chip in with a boring bit, Monarch are not legally Bankrupt, they are In Administration. That is different. The administrator certainly can operate what they care to. The best example of this continuing operation was BAF/Jadepoint in the 1990's. Creditors cannot seize assets, like aircraft, for past debts when you are In Administration, the whole idea of which is to keep going what is worthwhile.
However, separately, that is UK law. This does not apply overseas, and thus the aircraft are in danger of being seized against any debt there, for fuel, airport charges, whatever. Hence you cannot operate any overseas flights without this risk.
I also don't understand the cost. At this time of year there is generally quite a bit of return capacity. Not all of it, but one would hope they started by booking the remaining available seats on UK carriers BA, Easyjet, etc, who serve many of the destinations.
Separately :
I am more than a bit sickened by Transport Secretary Chris Grayling's whoop-de-do about how wonderfully the return airlift has been organised by his department. He just seems to read the self-serving briefings from his bureaucrats without thought. Not to make any political point (there have been other incompetences by other governments) but he seems to have no empathy for any of the staff, only for advancing the careers of his own ministry staff.