I for one agree with amfsook. It is a real pain in the !§$ to collect a TH RF set. Many cards hardly ever crop up on ebay. So, either you are prepared to wait a LONG time to scrounge together a set with each card roughly costing $10-15 or you satisfy your collecting habit and buy the set in one go.
That is why I think it is reasonable to ask for a bonus, because the buyer does not have to go through the endless hussle to collect the set. The buyer MIGHT pay a slightly higher average per card, but then you would be paying an above-average price for a card you still need to collect the set...so it may not be that much cheaper to buy individually.
Besides, it could very well be that prices go up even higher and $300-400 would end up being a reasonable price in the end.
It all comes down to availability. At the moment, you see more RoS and T&D foils than TH foils (at least half the RFs virtually NEVER are listed). Especially the T&D RFs are pretty easy to get, as they do not represent the cream of the crop from that set. I do not even want to think about how much a MW Gil-galad might have been worth by now
BTW, if anyone was willing to sell a ring of doom foil, I'd really consider it. *hint* *hint*