Airlines have got wise to the old lags "engineering" their dismissal or claiming the Bond was unreasonable for instance if a pilot is already typed and have taken advice from various council, including the pilot Unions.
If the Bond specifies what it includes, ground training, transport, hotac, sim, OCC etc and you sign the contract, you are agreeing to refund those costs if you leave, full stop.
Now, if you jump ship and don't honour the agreement, should it end up in court, the airline would have to demonstrate their costs where infact, reasonable, therein rests a problem.. Most of us would argue the sim is xx per hour, hotel xx etc etc, however, the airline could reasonably claim that part of their " operational" costs could be included as without their fixed structures and organisation, the training could not be completed in the first place.
If you don't want to be " Bonded" politely refuse to sign it and try and negotiate a solution, sadly, this may mean no job.
Of interest is the increasing trend towards " non reducing" bonds, the result being if you leave anytime during the contracted period of the bond, you pay the lot back. This method, should it end up in court, heavily favours the airlines as they could argue the " training" was completed in advance and without it you could not have operated at any time and they are writing off the costs after a fixed period. The reducing bond argues they are writing off the cost over a fixed period and opens the door to all levels of arguements about work contributions etc. For instance, is it fair for the bond to reduce at the same rate for two guys, one whom is off on sick leave then holidays against the 100hr a month company bike.
One small consideration is the cost of you defending the action against you, you can't roll over and play dead and if you have assets they can take a charge over you could be in peril. These big companies have huge budgets and legal support, for every 50p it costs them, it could cost you 2 quid!
Beware, a colleague of mine is still paying back his to a well know lowco after they got an ex-partie judgement on him, he thought burying himself in HK would be OK,,not so when you have a home in UK.
Lastly, come on guys, we are meant to be professionals, not a bunch of chancers.