Firstly, good chapter, hilarious plot by the nefarious Trio (Hannah,Hermione and Neville).
On the subject of Galleon exchange rate, I find that 30 Pounds to a Galleon (made of pure gold is reasonably good). I think the hubcaps comment was an exaggeration. A 2 cm coin in diameter sounds reasonable. If the coin is the size of the Sovereign, 100 Pounds to a Galleon is a good ratio. I have researched this and if you are interested in reading my findings read my research project. My FFNet bio is http://www.fanfiction.net/u/1052455/