As anyone who has worked for the US government knows ;^), there are ways around the cost overruns problem. I know of people who have received a job on a low bid of $20,000 or so, and worked it up to the neighborhood of $100,000.

One way to cover yourself in quotes is to be very specific about what you will do, e.g.,

I will operate my lawnmower on the grass in your front yard from the north side of your house to the edge of your property with the goal of cutting your lawn. This action (moving the mower from approximately the house to the sidewalk) constitutes a "swath" and you will pay me $2 on its completion.

I will continue to cut swaths on your lawn ("mowing your lawn") at the rate of $2/per swath with the following supplements:
*cutting grass around a tree, planting, statue, or furniture $1.50 per tree or planting;
*moving lawn furniture or toys $7@;
*swaths longer than 40 feet: $0.50 per extra 10 feet;
*grass longer than 5 inches: base price is $5;
*removal of special weeds such as dandelions or crabgrass $0.15@; *use of hand-scythe to trim edges $2 per linear foot;
*assistance from your 8-year old $25 per hour;
*stopping to explain to you what I'm doing: 1st time free, all subsequent times $5.

The blades of the lawnmower are 40" wide, and your lawn is 83 feet x 30 feet. There are 8 trees, and 2 flower beds. So the base price for my services is $65.00.

On the day of my appointment, we will agree upon the number of weeds to be removed, toys etc. to be relocated and whether you'd like me to hand-trim the edges. I will add those charges to the price.

Please also note that this is the "Freddy Krueger Service." I don't stop for small animals. I can provide you the "Have a Heart" service, in which I relocate all nests, work around the mole holes, and promise to kill nothing ant-sized or larger, for a 250% surcharge on the final price based on the above quotes.

Then, just make sure the blades on the mower are sharp, so you only have to go over a swath once.

Working from a quote rather than an estimate requires you to be more disciplined, or perhaps disciplined in different ways. I shifted to quotes-only for my work a long time ago and I've found my clients like knowing the bottom line up front, it's easier to raise my rates and, with set fees for certain services, it's also easier to make a profit.

And, while this may be a difference in the nature of my work vs that of many of you here, I find it much easier to build a cushion into a quote than to get a client to agree that it's their fault that a $2000 job ended up costing $2500.

Cheers!
Sarah