Hi
Prob not much, best bet is to ring your local galv place, they weigh it before and after galving and charge you based on the weight change, Its only £100-200 for a whole 110 chassis so a bulkhead will be much less, these were pre credit crunch prices though, BTW if you can give it to an engineering/fab firm and piggy back it on one of their orders it will be much cheaper...
Mike