A plastic tent? Why? If it was to contain sanding residue one can do that easlily with a shop vac connected to the sander. 5 gallons of paint is reasonable as is the $60/hour for labor. Did they give you an estimate before they did this? I assume they put 2 coats of paint on and that they billed you for around 23 hours of labor? If so, I think the labor charge is inflated as well. They basically charged you $89/foot which is close to what it would cost you to awlgrip the hull. I would fight it. Boatyards usually have a flat rate per foot to prep and paint the bottom.