Here are some valuable tips on bidding on a mural project.
Keep youreslf from being burnt. Advise provided by Harry Powers.
Welcome to post your own tips here!

"First - the value of a work of art is in its content and its artistic quality.
You should not generate ideas and designs on pure speculation. That is simply working for free without any provision for future payment. Who can afford to do that? You should present your past work and say - in effect - this is me and
this is my work. Pay me a retainer fee and I will design something of
similar quality for you and then we will know what we are talking about
and I can give you a cost estimate for completion. The word "bid"
implies that a design or a fixed and known set of parameters is already
set up and you are simply the artisan who executes a pre described and
measured "thing". All materials and requirements would be known and
specified to you and you would know what you had to work with.

Do not - repeat - do not let yourself get boxed into situation where
there is not a clear and contracted agreement of who is doing what and
what the due dates are and how much and when you will be paid. Your
responsibilities and the clients responsibilities should be clearly
spelled out and agreed in a contract before you do anything at all
except to meet them and show your past work.

You could give them a rough "outfield guesstimate" based on your past
experience of x number of dollars per square foot of wall space but they
should understand that is a preliminary non binding estimate.

A suggestion is:

1 Discuss your past work and show illustrations of it to the client.

2 If they like it - and you - secure a check from them which is at least 10% of what you roughly imagine the total work might cost. Make a letter contract with the client stipulating how and when you will present your "design".

3 Next, if they like and accept your idea, draw up a clear contract which says what you will do and when and what they will do and when. If they do not like your proposal ideas you will have been paid something for your work.

4 If you are to proceed make the contract (including the 10% you have already been paid) specify about one third of the agreed money in advance, one third at an agreed midpoint, and one third upon completion.

To repeat. Do not "bid" or commit yourself to anything until you have a
very clear understanding with your client exactly what is going to be
done and when and how they are going to pay you for it. You could be
damaged otherwise.

All the best, Harry Powers."
http://www.harrypowers.com