This can be a really difficult question to answer as it can be so variable.
It will be interesting to hear what other peoples ideas are and what they charge.
I can tell you for a couple of artistic jobs I have been involved with, it was just for the love of it.
They pay for the parts, I do the design, construction, programming, installation and and looking after.
I don't have thoughts of being ripped off, the fun and involvement has been all that I needed.
Then I was asked to mentor an artist in a high school situation to do with wearable art using Arduino, for the 14 hours of my time I invoiced for AUS$800. Again, it was a lot of fun for me and the kids. They even taught me a thing or two!
If someone from a commercial enterprise came to me and asked me if I would be interested in being involved in a more of a 'meaty' project, more akin to the industrial automation engineering I sometimes do, then I would propose a rate based on the rates normally expected in that industry and dependant on the complexity of the project itself. I like to leave a lot of room for movement on the way I like to be paid for my time. Mostly I try to provide a good estimate of the effort involved and attach a monetary value to that. I really like to see the end user happy and comfortable with the whole deal.
To give you an idea, for my industrial engineering, my hourly rate may be AUS$50/hr to AUS$150/hr, typically falling at AUS$90/hr for project work that spans a longer time. I have a lower rate of course for any travelling to site that may be needed.
If an individual hobbyist was seeking assistance with say for example an Ardiuno project, I seem to provide it free of charge, just as so many do on this forum. I guess it harks back to my days as a radio amateur where we did and still do that, helping our like minded folk.
I don't know if I would be interested in small project jobs, especially if they, the customer, is hoping for a super cheap deal. If I did that, then I may feel somewhat irritated with thoughts of having ripped myself off. There are other things in life to do as well, sometimes doing nothing is beneficial.
Paul