I wouldn't pay a monthly fee either.
I'm not bad with excel so I would use that if I had a big round.
Its just in the app world things are free or very cheap.
I'd make the app for anyone who needs to organise work in a round not just windies that way you get volume and keep the price low.
The apps that are free are downloaded millions of times and make money from adverts.
This is soley aimed at window cleaners and does more than what people pay 100 plus pounds per year.
There are many apps over 100 pounds.
I see what your saying about for other people too.
Looking at for wheelie bin cleaning and landscape gardners too.