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I'd go as low as 8.2mil... well from 8 mil to 8.5mil.
Oh and click the "show graphs" button below the picture of the item.
And below the try it on button.
It shows you the average price every day on the left and how much it sells per day on the right. :3

Imo, the letters are worth more than the items themselves.
Because you have a choice of which item you want. Therefore, they are somewhat inflated and well if someone just wanted a Horns Of The Demon they would just buy the actual item instead of the letter because they'd be actually "overpaying" for it.
But maybe if they just want the letter for some other reason (maybe they can't decide or are holding onto it as in investment item) then they'd probably buy it. Makes sense?

well, people use to go check the graphics before they complete the trade, to be sure they are not paying extra or to be sure they are not selling for a low price. and the graphics says people buy it for 7.5 to 8 mil.
so probably no one will trade it for such a high price (unless they don't care about how much they're loosing, i'd say)...
but hey, it's just my oppinion.