Not exactly 30 times 40K if you are talking about 6 months ? it would be half that wouldn't it ? I am not au fait with the arrangements between CTC and Monarch but the crews are not supplied for free. This arrangement predates 2004 by a long way, many of the initial CTC pilots are now Captains on the various fleets. Additionally there are significant additional training costs for large numbers of low hour pilots who need more resources. More training captains, more training sectors per individual. Nevertheless the cost benefits must still outweigh the negatives since so many low cost operators now do it.
Undoubtably there was a cost saving factor in recruiting so many low hour pilots, they are after all "low hour pilots". The people who are not thrilled are those experienced pilots from the military and other airlines who are now in direct competition for these same right seat placements. However the company no doubt had to look at what was going on in other low cost operations and compete accordingly.
Perhaps one day the situation will revert to that, where such low hour pilots are politely told to come back when they have aquired more experience ? In the meantime make hay while the sun shines. I believe that it is the insurance market that one day ( should it feel compelled to do so) will drive the market back to more "experienced" co-pilots.