Currently, Family allowance is not a means tested benefit. If you have children under a certain age, you get the benefit. So I'd imagine that it costs less to adminiter than those benefits where more complicated criteria are involved. And presumably, if you're on £100k a year, you're not going to bother queueing down at the post office for the relatively small proportion of additional income child benefit would represent.
So it would cost more to administer, and wouldn't actually save much.
Could it possibly be that Nick Clegg's latest proposal is more about establishing a precedent for slashing benefits than any real attempt to save money?
driving lessons in Wallasey?