I agree with this person:

"Write your own specifications regarding the equipment, style, brand,
model
etc. of what you want him to install. Describe all the details. Have a
floor
plan showing where to install everything. Detail what items are to be
on
each breaker. Dimmers. Switched outlets. And so on. When he gives you a
final price get him to write it on a copy of your specification sheet
and
have him sign it."

I can also tell you that you are not likely to get a written estimate
on small jobs. Something verbal or a number written on the back of a
business card. They don't have the time to do anything detalied just
for something like that. You need to specify exactly what they are
doing (how many receptacles, how many light switches, or even what
their ball park esitmate was.