Also have some cushion for unforeseen expenses that may pop up.

And there are always unforeseen timeline issues like redoing the floors and it takes longer then you plan to dry.

Secondly, so long as the numbers provide a hefty spread, unforeseen items can be accounted for.

Accountant advised us to form LLC so as to reduce the liability and protect our existing assets against any unforeseen events.

Shouldn't be unforeseen costs and you can budget accordingly as part of the expense of the deal. 

So if they give you a price on the job and unforeseen work comes up, how is the handled?

If it's going to be a gut project you have less unforeseens with a fire damaged brownstone because of how they were built (2-3 layers brick thick and hand laid stone foundations).

We had an unforeseen $5200 come from fixing the foundation so it's tough to push another $1700 for planks everywhere right now.

However, since many don't have significant cash reserves, that hundred bucks can easily be needed for some unforeseen future matter as well, meaning they may not really be getting that hundred bucks a door as they think. :)

The potential for poor quality work, overcharges or other unforeseen events lead me to do a lot of things myself (along with the classic justification to buying power tools -- "if I do it myself it pays for the table saw/nail gun/laser level/impact driver" -- signs of a power tool addict.)