these people are just doing their job... they are not his friends, nor are they doing anything to help him out as a favour...
£100 each is a ridiculously high amount - most people dont spend that on family members never mind staff!
try to find out exactly what was said, where he got the figure £100 each from and put a stop to it asap - if he does this, it will become expected an may even influence other residence into thinking they have to give the same
if someone actually told him this, then he needs to sacked.
as stated by others, a big tin of choccies or something between them, or a smaller one each, or a bottle each, would be appropriate