I would say $50 is a bit much - my personal opinion. I have only used a generator once since last summer! That one time was for an aircraft that was chocked far away from the FBO. Everytime I have gone to someone's home or workplace they have power there and are actually very helpful. 99% of the time they ask me if I need anything and are genuine about it. The water tank cost is foreign to me. I only do dry washes because that is how I have done them for 13 years. I figure this - you have to dry the vehicle anyway after pressure washing them down - so why not do it in one step?!?!? I hit a panel at a time with a GREAT cleaner/wax spray (very expensive, but worth it to me). Everyone is amazed by this product cause it last for about a month.
Takes me roughly 1 hour by myself to hand clean a vehicle and vacuum it out.
*Spray Panel/Panels , then with 2 MF's go to town on sprayed panels.
Simple and effective for me.
So when it comes to charging that much extra, I would say it is a bit much. BUT - you did say that you don't fully NEED the mobile service and that there is no one else around doing it - so maybe it is ok to charge that much. One thing I always think to myself, "You can always go down in price, but never up" So maybe start with $50 - if there are no bites, then drop it to $25.
Hope this helps?!?!?!??!