Tony Montana said it best:

All I have in this world are my balls and my word, and I don't break them for no one, joo understand?

People will throw out "bills have priority", but like someone else said, if you are that in need of relying on money for bills, negotiate better on the front end.

Needing money is not an excuse for poor ethics.

Reputation is important in this game. The person you snub today may be the deciding factor for more work/money at a later day. These worlds are small, and people that keep their word are remembered, and people that don't keep their word are really remembered.