I've bid on a number of jobs over the years, and maybe one out of ten comes in at, or under, the bid -- it just doesn't work, unless you know EVERYTHING beforehand, and something still could go wrong. Most dealers I've ever talked to have pretty much the same (or worse) story.

To remain profitable, you'll either have to start doing the exact same thing over and over -- not fun, and NOT custom -- or switch to a time and materials billing format.

Now, I generally give the client a *proposal* of what it *should* cost, if everything goes as planned, but explain that the final price will be based on actual time and materials. I've yet to have anyone balk at this approach, and if they do, I probably don't want them as a customer anyway.