I have thought about this for quite some time, I have a few bucks from early christmas gifts and was thinking about buying my first phat, but I am uncertain what effect the update will have.
THE FACTS:
as of now party hats not only require alot of IRL cash but most people don't even realise you must have several hundred mil just to be able to pay a few hundred IRL dollars for the privilege to buy a party hat in game at its market price.
If anyone every bought it say on RS forums for strictly rsgp, as you have said its several billion coins, made possible by junk trading.
Technically, when someone payed 10bil for a blue party hat, if they were skillful in junk trading they could rid themselves of there junk and get alot of the gp back, making the party hat purchase very close to its market value.
AFTER THE UPDATE:
free trade means junk will no longer be required, so a partyhat bought with IRL cash as they will be bought/sold here on sythe will just be a trade from on account to the other. This means the buyer will offer nothing, and the seller will not receive the phats market value in gold, they will just lose the phat, so this fact makes me think the IRL value in cash will rise, since now the seller will not recieve several hundred mil cash for his phat loss, he will just lose the phat, and if I was selling a phat for no cash, I would want more IRL cash. At 50 cents a mil, thats still several hundred IRL dollars difference.
With people who only trade with rsgp and on rs forums, I think in this case the value will go down. Because now the buyer, who used to recieve several billion coins worth of junk with the phat, will now only receive the phat, losing ever last coin he/she payed for it, so I would want to pay less if I didn't even have a chance to get part of the money back (i.e. getting rid of the junk for partial money back). Also, not everyone has the skills to generate bils of junk effectiviely and efficiently, so sellers will need to sell at lower rsgp prices because now anyone who has a phat can sell it straight up for whatever price they want to, because generating alot of junk is no longer required.
Again, this could be completely wrong, but based on what happens now and what will be possible after free trade is again acquired, this is my best "prophecy" of the coming rares market, both legitly (rsgp) and the black market (sythe baby!!).