I'll go ahead and say at least half of the users that have posted either cannot afford a phat and/or have never had one.
You cannot really predict the price of phats as there are a wide range of different things that can cause them to go up or down.
If you've noticed, after the bot nuke, everything pretty much crashed. Phats dropped around 30% in value. It is still a good investment as it didn't drop as hard as lower rares. Lower rares have dropped over 50% in value. Partyhats could drop just in much as price in the near future. So it could drop another 20%.
Anyway, a big factor to determine its future price is Runescape updates. Some updates causes them to rise, others causes it to crash (Bot nuke). Let's take Squeel of Fortune (Buyable) for example. People panicked and sold all their items that can be obtained via SOF. Items that temporarily crashed were Spiritshields. Others that dropped hard and still continues to would be bandos. Bandos was already hoarded amongst players and a lot comes into the game every hour.
Now, the panic sellers could have sold their items and bought partyhats. Or it also caused some players who did not support it to quit. That means a few partyhats were taken out of the game. Less partyhats = Increase in price right? No. It is all based on demand. New players have no way of being able to afford a partyhat. They'd have to do frost dragons for hundreds of hours. Less partyhats = increase in price only happens if the hoarders decides to in cash their Spirit shards to coins and buy partyhats.
tl;dr
No one can determine the future prices of partyhats. You'd need to know about the updates and know if people with hundreds of millions of spirit shards would be in cashing it soon. Starry has a max pile of spirit shards, if he sold it for over 50B RSGP and bought rares, there's no doubt Phats would rise.
With more and more bots slowly coming back, I predict a slow rise in them soon.
8 months is a long time however. The market can change dramastically.. You get things like inflation. Tbh I wouldn't buy a phat, I would just sell the GP now and quit for 8 months. It's safer, plus you get the money NOW.
Agreed. I like to think of $'s value constantly going up if you invest it wisely. Also, with bots making a slow come back, prices in gold will drop by the time you're back. Buy it now, find a way to make more money off it. Come back, buy gold from a seller and you'll be able to afford more than just a yellow.