When Mario Draghi addresses Germany's parliament on Wednesday on the benefits of the European Central Bank's new bond-buying program, the ECB chief has a problem: it isn't clear how much his new toy will cost. The good news: it might be less than the last one.

The new program, known as Outright Monetary Transactions, is theoretically "unlimited," to discourage markets from betting against it. In practice, the OMT has more built-in restrictions than its predecessor, the Securities Markets Program, under which the bank spent some €210bn.