Depends on the program, but generally it ranges from 0 to 20%, most being right around 10%. This changes when republicans win on their "voucher" planks, when suddenly the overhead skyrockets because there are for-profit entities in the loop.

As I understand it, the idea of a voucher is that the for-profit entity receives the money after the end beneficiary does, which wouldn't affect how much the end beneficiary gets. What are you talking about?

Some programs, like schools, dispense services rather than cash. If you're talking about direct payment programs these are extremely efficient: 4.5% for SCHIP in 2005, for example. Other direct payment programs have higher overhead because they are means-tested (another conservative victory) and it costs money to administer means-tested programs.

Amusingly, the US state of Florida in 2011 enacted another one of those republican favorites, the drug test for welfare recipients. In practice it cost more to administer the drug tests than they saved by throwing the positive results out of the program, because it turns out that welfare recipients are far less likely than the general public to use drugs.

Republicans seem to despise moochers more than wasting money, despite all their claims to be doing it to save money.

A lot of "voucher systems" (a) impose overheads, which are generally paid by the donor and not the participating company or (b) use hidden, non-open-market (and potentially higher) pricing.