Seems like an incredidibly narrow-minded way to gauge its economic value to me.
Well, we could do some one-off studies and hope that Excel is used properly, but why? We already have mechanisms that figure out the value of things and allocate resources appropriately, they're markets. The unviable nature of public transit suggests systematic resource allocation problems, created politically. If there were really such huge beneficial side effects, it wouldn't matter if ticket prices rose a lot, that'd just get priced in to other things like cost of goods near transit stops.
For instance maybe every ride in NY should be 2x or 3x the current price for the costs of the system to be worthwhile, but public transit is usually government owned and politicians aren't willing to bite the bullet on that.
I don't disagree that it's hard to quantify the exact economic value, but you can't use a free market to solve coordination problems like public transit.
Coordination failure in stages:
1) A transit line that goes where you want, when you want for only a fraction of your trips means you need a second means of travel the rest of the time.
2) Having a second means of travel on hand (e.g. a car) means that you're less likely to use the transit line when it would work for you.
3) Lack of use of the transit line leads halted growth and reduced service.
4) Reduced service further drives users away
5) Evidence of failure causes a loss of investor/government support.
6) Transit line dies slowly to the extent that political inertia allows.
Or to put it another way, asking a transit line to pay to for itself is like saying "if human beings really wanted world peace they could have all just disarmed!", ignoring the fact that until everyone else is disarmed you'd be a fool to disarm yourself.
And if you succeed and build a critical-mass, sustainable system you've just swapped your market problem for a natural monopoly and have to use non-market techniques to deal with the monopoly service provider. (And now we're back to NY)
