100 Galleons is a huge fee, and Harry's been spending his parent's money on school supplies and robes for five years now.  Lily and James weren't actually out of school and married for that long before they had Harry and died, so I always thought they didn't leave a terribly large ammount of money in there, just a few years earnings between them (let's assume they owned the house that Voldemort destroyed when he killed them and that they also had to pay other living expenses with their wages while they were alive).  It was an integral part of this story that Harry didn't have enough money to go.

Hope you like the rest of the story :)