Just washing the exterior, assuming no detailing/waxing - Maybe $5-10 unless you're a highly detailed expert (ie, q-tip detail.)
If you're just picking up the trash and wiping the interior down with water, for a 4 door 4 seat car, $20.
If you are actually detailing the interior with product, cleaning the carpets with shampoo, I don't know... $50 or so.
Of course, I only charge my friends lunch or a couple of beers to help them out. And that includes buffer work that might need to be done. I mean hell, they're friends - not customers.