Airberlin advertises that if you book the flight via their webpage you won't get it cheaper anywhere else. As a topbonus member I went to book a flight to London from Germany but before booking took a look at the British Airways site. The flight in question is operated by BA but it is code share flight with both a BA and AB flight number. Now I know each airline is responsible for its own prices etc. but I was still shocked to discover that that the BA flight cost exactly half the price of the same flight with AB. Same conditions (except for less Topbous points) Are these sort of price differences common? How do they expect to stay in business? I would maybe have been prepared to pay a slight premium for the extra miles but 100%?

If it's a code share both AB and BA probably have a stock of tickets that they sell individually. And if there is a difference in mileage earning it is also a different booking class with different fare rules.
If you can find an AB ticket on that flight for less money, it's a different story. But you probably cannot compare different airlines' offers on code shares.
So, be happy you found a better deal and book with BA.

Cheers,

-txl.

That is how the industry works. Each is responsible for marketing and selling their seats.