Depends on the competition.
If there is only one signature campaign, it could get away with paying almost nothing.
I'm not sure that's true. I mean, having a paid signature these days will ensure you receive greater attention from mods here, as well as some users/subs ignoring you altogether; I'm not sure most people would go through it, if the rates weren't worth it - at least, I know I wouldn't.
Yes, that is true. Some users might think it is not worth their time.
There would be others who think something is better than nothing.
Still, I think new signature campaigns look at what the existing campaigns pay, and then accordingly benchmark their rates.