The real problem here is quantifying positive externalities associated with the benefits from a "pretty bridge" to determine if they are worth the money. They are real, accrue non-uniformly, and are hard to measure. The users of the bridge aren't only those who cross it. Tourism does produce real jobs, even for those who never walk across. As Redding grows, $20 million could be a very good deal.
You can get a "pretty" bridge for $2 million. No way is your bridge going to generate the other $20 million. You are just trying to justify a government boondoggle.
Tell that to the workers at Turtle Bay Park that are being layed off.