I think a good way to measure the benefit is to ask how many tables did it put food on? How many households did that $2.6 billion support (not by way of hand outs, but by way of creating actual productive employment, e.g. not government jobs)? etc.
I don't know the numbers there, but it seems to me that putting an asteroid in orbit around the moon probably wouldn't support as many households as something like having an economy that actually supports manufacturing jobs rather than eliminating them.
As for NASA doing something more productive, well, I think it's pretty hard for them to be very productive as they're really more of a research & exploration operation. I suppose the question is whether they can create productive employment from that research. Seems tough though. I don't think licensing logos really is much of a productive business. Would you pay $0.50 more for "NASA Lettuce" at the grocery store for zero benefit? Then again, that might be a better way for them to fund themselves rather than getting tax payers to pay for their work/antics.
If people really believed in space exploration all that much, they'd be free to fund it. Get yer NASA hot dogs! $15 each!
You'd only need to sell about 217 million hot dogs at that rate, assuming around $12 of it goes to NASA. Call 'em space dogs! Or moon dogs! Of course at $15 per hot dog, that makes them pretty much unaffordable for the roughly 25% or so of the world's population that needs that $15 to last 1 or 2 weeks. I don't think they'd be all that interested.