you have to basically gamble £7K on an mcc which costs £2K anywhere else, and then you still could be told 'no thanks'.
A few other things to bear in mind:
When I went through, cost was £6.5k, but included £1000 for acommodation for the course. If you sort out your own that will reduce the cost.
You receive £2k back on completion of line training (don't know if this is still the case).
The AQC is not just an MCC. The course includes a JOC on top of the MCC which is a week of manual jet flying which is a very good experience to have prior to starting a type rating.
As someone else mentioned, 90% or so of people selected for the AQC get through - they don't put you through from the interview unless they think you will pass it - all 16 on my course got through 18 months ago.
Bear in mind though that the contract is only for 6 months flying - there is no guarantee that you will be kept on after that - although almost everybody going to Easyjet and the like get permanent contracts.