Where does the money go?
NASA makes their request for a budget amount, complete with justifications and such to Congress. Congress looks it over and approves or attempts to amend based on their decisions. The resulting money from the budget is used to fake the subsequent missions and plans they had submitted to Congress, and given that it costs much less to fake it than actually produce the real results, the higher-ups can profit the difference.
Financial auditors are going to look at invoices, they are not going to take the word of NASA that they spent the money on this and that. For example, rocket fuel costs, power requirements, etc. Auditors will account for everything.
In addition, though several different contractors may make different parts for a rocket, NASA does not construct the rocket, it is designed by a contractor, who subcontracts for additional parts and labor.
I happen to work for a govt. agency, not NASA. I am a network engineer, however I do not make the equipment used to run our network, it is provided by a vendor, who orders parts from other companies for parts such as circuit boards.
The F/22 raptor is made by Lockeed Martin, for example.
http://www.lockheedmartin.com/products/f22/Also, to respond to posters saying that financial auditors do not know the technology, thereby they can be fooled, you are correct, but then you also have security auditors, infrastructure auditors, etc. They do look into the technology, and they do know what they are talking about.
If individuals at NASA are pocketing the rest of the money that they do not use, then how have all of them (over 50 years of NASA execs) evaded an IRS audit? Where would they put all those billions of dollars? Not the bank I hope.