Well being a "profetional welder" I am cwb structurally certifed, in most process, and I do a little onsite every now and again.
I would not have come out in a lightning storm that is rediculous.
We charge, $150 booking fee, just to get us onsite, and $70/hr service.
Keep in mind that we have a complete outfit, with everything we need, including tickets, and this is for structural service not just fixit work.
The big thing is not only taking in account for your time, but for your materials as well like rods, or wire, and gas, and gloves, helmet , welder, grinder, grinding disks, ect... Sure lots of people can weld, but not many have the equipment.
I think $25/hr is very reasonable for a well done job, infact the absolute cheapest shop I know of is $40/hr and he's on his way under. For $18/hr they better be suppling everything and all you do is show up and burn rod, and grind.
Hope this helps some.