Compare NASA to, for example, Xerox PARC (Ethernet, the GUI, laser printers, etc.) or Bell Labs (the transistor, access control lists, UNIX, etc.) and see which produced more inventions that benefitted the economy as a whole per dollar spent.
Each shuttle launch cost, on average, $1.5bn. The cost of one launch would fund over ten thousand PhDs, or several hundred DARPA programs. Do you really think that NASA is the best ROI for taxpayers?
The problem with NASA is largely the senators dictating how the money will be spent, which leads to a huge amount of wastage. The shuttle is a good example - NASA could only get the funding if they made a space craft that fitted some fairly mutually exclusive specifications - the result was a space craft that could do none of those things especially well and almost certainly more expensively than building several separate craft tailored to specific jobs.
Look at the A-3 test stand as another example: it was designed for the Constallation programme, and when Obama cancelled the programme the partially constructed test stand was of no use. Congress demanded that NASA keep constructing this useless piece of hardware and they spent about $200M on it _after_ it was known that there was no use for it. How can you expect NASA to be value for money when it is treated as a jobs creation programme and forced to waste money like that?
SLS is probably another good example - insanely expensive, not least because congress are actually dictating the engineering requirements, and no doubt the government will order NASA to scrap it before completion, completely wasting all the money that was invested in it. Despite its huge cost, I kinda hope that SLS doesn't get scrapped, because then at least the money has gone into something that can be used instead of yet another useless cancelled project.
Far better would be to just give NASA a lump of money and tell them to do with it as they please - the money would still end up invested in paying people to do jobs (the jobs might not be in the various senator's chosen locations, but they would still happen), and we'd probably have a lot more science at the end of it instead of a huge pile of half-completed scrapped projects.