I guess party hats are being lowered because not as many people can afford. When staking was out, people could easily afford etc, now money is harder to get.
Not really the case anymore. If you look around mostly 2/5 of the level 100+ players in RS have more then 20m now and with godwars and other things out lots of players obtain GP easily.
But as for phats i think that to many people are trying to sell them when not enough are buying them and with jagexs market its hard to do lots of things but they will raise one day once they are mostly sold out.
JaGex has lowerd the party hat cause they do not sell, they buy, but once you get them, your stuck with them. There trying to make it so more people can buy party hats, like when santa was 10m, so many people bought santa hats, there just trying to get players to spend more time on runescape and make more money, so here making big things famous