It took me a few years to get it right. It depends really on the arear you cover and how well you know it. We cover a large area now that I know very very well, so if someone calls from an area made up of large and different houses I know I need to see it. On the other hand most of our area is the same really, large new build estates and so on. I find our prices are spot on and if someone picks the 12 week option then that job is very well priced as the 6 week option reflects the true price for that house.
I think there is no need to have a fixed price unless you are a bigger company. As a one man band I wanted to see everything. I only came up with the price menu when I simply couldnt keep up with the growth, I became the thing that restricted it. Now were onto 25-50 new houses a week the menu is a god send, you could never get round that many.
So to some up it depends on what stage of your business you are at and what the area you cover is made up of and your knowledge of that area.