Yeah, my minimum is $40 and I make 2 exceptions. The $25 one which is like 1,000 square feet and a $30 one that's really just a grassy parking area for a guest cottage that parks like 8 cars. They get a break because it's small, I do their main house too(they call it the Mother Ship, lol) and I get a lot of other work from them, over $3,000 this year to date.
There's a certain amount of time that goes into maintaining any account...customer contact, billing, collection, record keeping and of course getting there and firing up 3 or 4 pieces of equipment. When I started out 11 years ago I set my minimum at $30 but did offer a 10% senior discount, so I had some at $27. Over the years I bumped it to $35 and then $40. I really don't want any accounts that gross less than $1k/year although I have about 6 bi-weekly accounts that do.