It depends on the book and the cover. First of all to have any real value it should be a first edition hard back. Paperbacks are not going to get as much money. The real money is in original copies for example a first edition hard back of the philosophers stone with signature could be worth £8000 or more!
Right now, the market is so flooded with copies of Harry Potter that they are not worth very much at all. You can get used copies online for just a couple of dollars. If they are autographed, special edition, or have something else interesting about them, it could raise the value.
Not much. They aren't real and you should make sure the person buying them knows that.
about $2 a card
Medallions are not part of the magical currency, they are not in any way directly related to Harry Potter values and as far as I am aware they are not mentioned, thus have no mentioned worth
Usually it is $98.92 but on amazon, it is $74.99
about $2 USA currency. I dunno about UK.
200 or 300 dollars
it depends on the picture and the amount that was released to the public. just like a paintings etc.
depends on who signed DH
$1000 because right now 1 direction is really popular and their fans will do any thing to get a autograph or meet them.
it is worth about 250,000
it is worth a £10