If you plan on charging Rush Charges, you had better have some type of reference point. That is, how long it takes you to do a particular thing. I'm a freelancer, and I only have two hands, so it isn't as if I could magically make myself go faster. So, if it takes me three hours to draw a cartoon, I charge for three hours.
Of course, you could have Rush Charges that could knock you off of working, at least temporarily, for a client who had been there first. But watch out if you are messing with the *First Come, First Served* principle - it's pretty darn important to people. And if you get a reputation for doing that, well, it's not a good thing. Essentially what you're saying to your other client is that you're gonna allow someone to cut in line in front of them because someone else is paying more money. That's not nice.
But freelancers do sometimes raise their prices, and I'll let you in a little secret. They do it because they don't want the job. It's a technique I learned a loooonnnng time ago, to never say no to a job, if you can actually do it, but don't want to, just quote a very high price. Now waitaminute here, this isn't a bad thing. What is does is to chase away the time-wasters who are screaming that they would do anything to get the work done, and give an opportunity for the genuinely sincere people, who are willing to pay a premium. Every freelancer knows about doing a very high quote, and expecting to go fishing that day, only to find that the client agrees, and wants them to get started right away. Yes, I will do that! It's brutally hard, but it pays well, and clients who have the budget are very appreciative. Of course, if they don't accept the quote, you do get to go fishing after all, and no hard feelings.
I hope this helps. Doing freelance work is a business, and you have to learn to be business-like. I took some business and accounting classes when I first started out, and they were as important to me as any of the other classes I ever took. Whatever business you're in, if you want to earn money doing it, you have to learn the business of business.
Posted by Brad Hall