I think you've got to use some common sense with regards to having a minimum price. A friend of mine does many of our town centre shops and some of those are just a few quid, ranging to some nicer jobs, but they'll be for a few windows and the work is weekly mostly, and extremely compact.
He can start work really early and by 10.00 am he's turned over a good wadge; in any weather almost.
Also if you do rural houses at £25 a minimum price, and I have quite a few days work like this, it would be daft to try and charge £25 for a small bungalow which you know you won't get if that property is adjacent to a current customer whom you're charging £25+ plus for.