The other comment makes some points, but the bottom line is that the purchased bonds pay higher interest than the Fed Funds rate, and the value of them has also gone up a lot as long term rates have gone down. So capital gains and income over cost of funding.
The capital side might change, as bonds may fall in price before the Fed sells them.
Well let's examine the overall. I'd say for now it has been immensely profitable, even though I dislike the program.
1) QE has held down the cost of US debt, probably saving a trillion plus in interest payments the last five years alone.
2) QE has devalued the real value of foreign held debt. Eg China + Japan's $2.5+ trillion in US debt they're holding has lost a lot of real value courtesy of QE. That's less that has to be paid back in theory (it won't be paid back in reality). Saving hundreds of billions regardless.
3) A lot of dollar assets are held overseas (and of course commodities trade in dollars), but the primary beneficiary of the dollar-based asset inflation program has been the US stock market and US real estate market. Foreigners are partially subsidizing, via the dollar global reserve standard, a huge rebound in US asset prices. It's a form of redistributing wealth from the rest of the world that uses dollars, back to Americans that own real estate or stocks.
In effect while the economies of Japan and most of Europe have been in ten year recessions in real terms, they're paying for the US economy to be doing relatively very well right now. All partially accomplished by the US Dollar standard, that can be freely abused by the Fed to prop up the US at the expense of the massive foreign dollar holdings.
4) QE has been partially paying for government employment, welfare benefits, social security, military employment et al. If the US couldn't have used QE to fund the huge hole in its budget, the US Govt. would have been forced to divert a large amount of money to paying higher interest to draw in buyers for that debt (or the US Govt would have had to slash its budget big time). Instead, the US was able to keep people floating in jobs or with food stamps or whatever, people who otherwise would not have been ok at least near-term, and the net cost of them not being ok would have been even higher long-term than the cost of the last five years.
5) The US residential housing market alone has gained something like $10 trillion in value since the bottom, due in part to the Fed's program of holding down interest rates. That gain drastically offsets the cost of QE. However, QE also helped inflate the stock market substantially, according to the Fed's own words on the matter, so there's a few trillion dollars more. The asset inflation side of QE has been wildly successful so far - the primary question is whether everything will ultimately have to crash back down, post QE and or with higher interest rates.
