A recent CNN poll summarized here found that Americans tend to be wildly misinformed about federal spending.
For example, what percentage of federal spending goes to pay for public broadcasting? Half of those polled thought it was at least 5 percent. It's really closer to one dollar for public broadcasting out of every $10,000 of federal spending, or 1/100 of 1 percent.
On foreign aid, education, food assistance for poor Americans, and government pensions, the median guess for each was 10%. It's actually about 3.5% for military and civilian pensions (and other retiree benefits) combined, 2.8% for food assistance, 2.7% for education, and 1% for foreign aid.
Housing assistance for the poor? Those polled guessed a median of 7%. It's actually just 1.7%
For more of the survey respondents' average estimates versus the actual numbers, see this table. (Note, however, that on the second page of the table, a majority of respondents favored cutting only two categories of spending, government pensions and foreign aid. That is, they'd apparently be happy if we were spending a lot more on the other categories than we really are.) The complete poll results in can be found here (PDF).
Surprisingly, when asked about the cost of Social Security, the median response, 20%, was correct to the nearest percentage point.
Unfortunately, there are plenty of other things about Social Security that people get wrong -- including CNN, which suggests that cutting benefits is a good way to reduce the deficit. In fact, Social Security is financed by its own dedicated tax and borrows no money, so it's highly misleading to suggest that it contributes to the deficit, and it would be grossly unfair to target it for cuts.
There are so many ridiculous myths and quarter-truths about Social Security -- e.g., that it's a "Ponzi scheme," that it's projected to run out of money and stop paying benefits, that Congress evilly "raided" the Trust Fund, and so on -- that I'm going to save debunking them for another post.