I don't run a big shop so my opinion is probably not valid, but when I bring in help I have a per service job rate I pay. Typically I'll be doing a paint correction and I bring in a select couple guys to do the interiors while I work the outside. For just basic interiors and help with washing I allot $25-$30 per vehicle and then on more in depth steam and shampoo or high end leather clean and treatment work I will budget a little more. I also base it off of experience level and difficulty of the job. My mood for the day can also change the rates lol. It's not very common that I bring in help though because I typically only have one vehicle at a time.