My son (8) went through this last year. They're quite expensive. He lost interest when they were no longer allowed to take them into school. He traded a couple "normal" cards with one boy for some special card or other, and when we looked up the card he had got, they were going on
ebay for £30. So we made my son give it back.
Not sure where his cards are now unfortunately. Maybe there's a dedicated forum somewhere where you can trade?