I'm a freelancer and I currently have a full time (well, 32 hours/week) gig that'll probably last at least until the end of the year. I've also been asked for some occasional consultancy on the side for another client, which would probably be only a few hours each month. I don't know the details, but it seems to involve checking to see if a (team of) more junior programmer(s) is on the right track, and maybe offering advice or something. Could be interesting, but I have no idea what to charge, and I'd like to have some idea before I speak with the client.
I'm making plenty on my regular gig, and I don't really need something extra, but it might be a nice entry into a different (even better paid?) way of working. I have done something slightly similar a while ago, and then I charged my usual rate (€70/hour), which turned out to be way too little; even for a few hours, it's still a lot of hassle, and ending up with an extra €150 is just not financially interesting. Besides, because I already have a full-time gig, it's basically overtime for me, isn't it?
So should I ask double my usual rate? 1.5 times my rate? More? Less? Way more? I really have no idea.
Also: what do I say to justify this higher rate to the client?
This is in Netherland, in case it matters.
So I'd do an intake meeting with them to get a sense of their needs. Ask lots of questions, do your home work and figure out what they need. Then draw up a scope of work that outlines the work you're going to do and put a quote on that.
Historically all jobs suck equally and they work you just as hard, so you might as well be compensated well for it and if it's too much and they balk, then it's fine.
Someone once gave me advice that said if you really want to work on a project charge a lot and if you don't want to do it, charge more. I don't necessarily employ this advice, but...it's an interesting thinking.
You don't have to justify anything. You're a professional who knows his craft and if you outline your recommendations and what you're going to do in writing -- which is what a firm would do -- then you're going to be fine.
Realize if it's too high they'll go with someone else, so decide if you REALLY want the work or not. If you want the extra money, then decide how big the project is and come up with a total price and then divide that by how many hours you think it'll take realistically (in other words, more hours not less hours) and then quote them based on that hourly rate.