It really depends on what they ask for and the situation at the time. Last year it was about £100 each - this year, due to unemployment in the house, it will be much less and something very inventive ( I considering turning my boys room into a Seagulls den, re-decorated, new Seagulls bedding, players photos on the walls as well as flags from the Doncaster game and the Albion mosaic properly framed ). Not too much financial outlay but more time costs.
It depends on what they ask for, and how much we have spare, but probably about £100 each, once Father Christmas has been factored in. Obviously, the kids budget comes first, and we end up spending what is left from the budget on ourselves.