Well, as of now it's about 5-7k, but after the 1.5 update, with the melting of netherrack, price might get a bit lower.
Yes but it would cost a fair amount to smelt netherack so I don't think the price could go too far down
Why would the price go DOWN for more uses? Netherrack now: Infinite fires. Netherrack 1.5: Infinite fires & netherbrick. I think that with more uses, it will actually gain some more income.
Netherbrick has varied widely between around 120r-200r/stack. In some cases, it can go lower or higher, of course. With the 1.5 update, 4 stacks of netherrack will be required. Netherrack has a low value (around 10r/stack) now. So 4 stacks would be 40r. Plus 32 coal at around 2r each = 64r. So the new price should be 104r + mark up for the time to smelt. So yes, prices will fall, but not dramatically. What may also happen is that the price of netherrack will rise, to keep the price around the same.