How much does a 3 day cabin crew recurrent training cost?
The airline will bring in contract instructors to do the training (yes there are freelancers out there that do that sort of thing as contractors).
If they don't have the number of active crews to justify full time instructors because they only need someone for 5 or 10 days a month, that's the normal alternative.
The airline will still be paying the contractors they hire for the training so the cost to the crew will still be zero.