I dont know the exchange rate so I cant tell how much you are charging. But it looks like 50 euros/car. Im guessing euro now is somewhere equal to $2.00 U.S. Do thats around $100 U.S. Just on the lower side anyone here would do a lot to a car. Probably a wash,followed by an All In One product like
Poorboys Polish with Sealant. Also washing engine to remove the crud, but not detailing the engine department. Interior would be nice vaccuum and wipe down of hard surfaces also cleaning the windows. Tires would be cleaned and treated to a quick application of tire treatment to make look better.
Its just hard to do the money conversion. Also there are some low water usage products out there that could be used for exterior cleaning od the cars paint. Such as
Poorboys Spray and wipe (which I have), Optinum No Rinse is another I can remember being talked about (I dont have this nor have tried it). You should be able to get the
Poorboys World Products over in France from a Poorboys distributor in Europe. Read the directions or ask questions on any products you may need help with.