I've experienced the exact same thing time and time again. I get busy, I throw some huge number out there, they accept immediately, PayPal me within hours, and are the best client. WTF. I recently had a few clients need rush / weekend work completed too. So I doubled my hourly (they're long-term clients, not one-offs) and guess what. They accepted.
I can also attest to blackbootz and insomniasexx's experiences. I was recently approached to do a piece of work that was very much outside of my comfort zone and a rush weekend job. As such, I just went in with a crazy high price. In the grand scheme of things it wasn't that much, but I don't exactly have a massive portfolio and it was more than two times what I'd charged previously. They accepted. Unfortunately, I think that particular project has gone south despite me delivering something that I think was pretty good for the time and terrible brief they gave me. At least I got paid I guess, but I feel a bit bad about it. It was mostly poor communication from their end, but in hindsight I should have pressed to get the information I needed. The lesson I learned: Always make sure to get a robust brief from the client. Don't let them be vague because you will end up failing to meeting expectations you didn't know they had. It sounds simple now, but I suppose the obvious things are the easiest to slip by in the beginning.