back on my first account in rsc the rarest party hat were green ones, they sold for 6 million gp but the reason why party hats are so expensive is 1) they're rare and 2) they are a purely status symbols. They were only dropped once on christmas of '02 and Jagex decided on Easter of '04 they quit dropping tradable holiday items because the purpose of the holiday was not to make money so since then there have been no more tradable holiday items and Party Hats are now the most expensive holiday item but originally they were the least expensive selling for only 5 gp a piece. Any way thats my two cents i hope it was helpful in some way.