Now, some other things came to mind as I thought about this.
In my area we have to have a business license for where we operate out of and sometimes the city where we are doing business in. We also have to have a Registrar of Contractors License that is part of State control. Then there is bonding. You might want to be careful about these 'side' jobs if they are very big. You might find they cost you more than they are worth. I say this because I know many of you are pipeline, rig, and other various rigged out welders that normally don't do this kind of work in your local home area. Be Careful.
That doesn't mean don't do it, just watch your back.
So, another point. Because you don't have this kind of overhead, nor a shop facility to maintain, you could probably justify working cheaper than the $75 that I mentioned for me, and even the $65 mentioned by others. BUT, don't give your experience, knowledge, overhead, and skills away. You are worth more than that, especially on the jobs you normally do. And, if you go too cheap, you ruin the market for those of us who make our living day in and day out jumping through our own hoops and trying to make a living. The overhead for facilities, equipment, insurance, bonding, incorporation, Registrar, city licenses, certifications, updates for the city building authority, office staff, code books, and so much more make our job just as difficult as yours, only in different ways.
The cheap, backyard mechanics, with no insurance, licenses, etc are there bothering our work load as well. We all have guys who try to cut in without any of the proper preparation to really do the job right. Don't be part of the problem. We are all hungry and trying to get any work we can. We have to stick together or we will all fail. When you work too cheap, eventually it will catch up with you. It either means you will get caught without everything required for the work, OR, you will not be able to renew all those needed items when they come due even though you were able to put food on the table today.
Hope I said this in an informative and non offensive manner.
Have a Great Day, Brent