Since the announcement that Stars bought FTP, a lot more people are trying to sell their FTP money. There's definitely potential for buyer and seller to benefit, the seller from getting needed money immediately, and the buyer through paying a low enough price that he gets a good deal. Here are the factors affecting the value of FTP money:
Time preference. Money now is better than money later.
Risk of Full Repayment not Happening. For ROW players, it's very likely that repayment will happen in full, even if there are minor delays beyond the timeframe given by Stars. But it's never an absolute certainty until it actually happens. And for US players, there's a greater chance of further delays or lack of full repayment.
Risk of Player Scam/Hassle. The other party may delay or outright scam. With reputable people, this risk may be very low or negligible. But this is one problem for sellers - even if your plan is to snap-ship FTP funds the minute you receive them, the buyer is going to account for whatever they perceive the risk of scam/hassle in their offer.
Premium of Cash over Online Money. Stars money trades at a discount of at least a couple percent compared to bank money. The buyer of FTP money may be trading his BOA/wire/Paypal for what will (if everything goes right in a ROW FTP account) be online money.
Based on the above factors, what should FTP money trade at? Personally I'd rather Merge money than FTP money from a reputable ROW player, and Merge trades at 87% or so. So if I had to come up with #'s out of my head, I'd put reputable ROW FTP money at maybe around 5% lower than Merge, say 82% or so. The DOJ concerns for US players are serious, so I'd lower US FTP money by another 10-20% and say 67% or so.
I'm not sure how many trades are actually going through or at what prices, and I'm not necessarily buying myself even at these discounts. But if I had to estimate "fair" prices, those are roughly the figures I'd give assuming reputable sellers.