I've found a solution there. The problem is a typical one from other programming languages and still as easy the overlook in each one.
In effect what I'm doing there is adding up a number of floats, but if the field isconfirmed is <= 0 I'm adding an INTEGER value (0). As is in many other such situations (in different programming languages) a conversion happens then. Thus the Integer value in the SUM field leads to the whole sum being seen as INT. Although what is still a bit of a surprise there is that it seems like that even the partial sums get then converted into INT values (at least that is the impression gained from tests).
The solution is now quite easy there and completely fixes this problem:
=Sum(IIf(Fields.IsConfirmed>0,Fields.Cost,0.00))
The 0.00 leads to the zero value being interpreted as a number with decimals and thus no int conversion happens.