I like OgNasty's proposal, although I do agree with others that 2BTC is a fairly high barrier. With my current sig campaign, that would take me 20 months to earn. I think something between 0.75 to 1BTC is more reasonable and more attainable.
Wrong. The more you lower the amount, the more spammers you are letting through. As a normal user that tries to engage in discussions and help others, you don't need a signature (if you can't pay for it).
I think having a lowered amount could essentially work. A tiered system would get draw in those who only want a sig without the idea of promoting for profit. 1 BTC for a line or two of plain text, no bells and whistles, and 2 BTC for the more extravagant looking signatures that higher ranks have.