What are your setup costs? Insurance, as stated above. You need good quality products to do the washing, a couple of sponges and buckets, a strainer, clothes, polishes, glass polish and clothes - and then if you're doing inside too you need loads of cleaners for cloth, leather, plastics. Wheel wash and clothes just for that too.
Also, how are they providing water - in your bucket or via hosepipe? If it's in a bucket you'll need them to keep going in and out of the house (I wouldn't let a stranger in to keep using my tap), so it's not really saving them any bother. For 5 they can just take it to a hand carwash.
Finally, unless you've got a van with a generator (and now you're into thousands of pounds), you can't vacuum so you'll only ever do half a job.