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.
I also think that we need to be harsher on campaign managers to enforce their rules more strictly and have stricter rules about spam. I think that punishing managers for their campaigners' spammy behavior would be a good incentive to get them to be stricter. Perhaps giving out bans to managers if their campaigners are being spammy and nothing is done about them after they are informed of spammers.
The amount of course will have to vary based on the USD-BTC exchange rate. Signature campaign payments are linked to USD and of course, this payment would have to be linked to USD as well.
Not all campaigns are linked to USD. Many campagins, such as betcoin's, are Bitcoin only. This forum is Bitcoin only. I see no reason as to why the rate of the payment should be linked to a fiat currency. If it were to be linked to something, I would rather that it be related to the campaign rates and only change when campaign rates shift dramatically, as they did with the last big increase we had a few months ago.