How long will it take is the question? Over an hour and it becomes marginal at £30.
I have a small block of ten flats including communal entrances. Which I charge £53.50 on every quarter. It takes me an hour and twenty minutes plus one invoice which takes about 5 minutes to produce plus a stamp and a walk to the postbox.
So an hour and a half of which ten minutes is in the evening during non-productive time. I think it is nice work but wouldn't drop below £50. A fiver a flat every quarter is good value for the custies and for me.
No, I lie ... if I was pushed I might do it for £45. I can do it in the rain. Or start at 8.00 am. Or after 5pm. That's useful.