Well, here is my test code:
Qt Code: 
  1. tmpOrder.iMerchandizeID=1;
  2.     tmpOrder.iMerchandizeQuantity=25;
  3.     tmpOrder.rMerchandizePrice=4.00;
  4.     tmpOrder.rSubtotal=tmpOrder.iMerchandizeQuantity*tmpOrder.rMerchandizePrice;
  5.     tmpOrder.strMerchandizeName=QString("Testni Artikel");
  6.     tmpOrder.strDisplayString=QString::number(tmpOrder.iMerchandizeID)+strMerchandizeSpaceDelimiter+\
  7.                               QString(tmpOrder.strMerchandizeName)+strMerchandizeDelimiter+\
  8.                               QString::number(tmpOrder.rMerchandizePrice, 'f', iMerchandizePricePrecision)+strMerchandizeSpaceDelimiter+\
  9.                               QString::number(tmpOrder.iMerchandizeQuantity)+strMerchandizeSpaceDelimiter+\
  10.                               QString::number(tmpOrder.rSubtotal, 'f', iMerchandizePricePrecision);
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Now the float values (price per unit and subtotal) are ok, but as you can see I do not use .arg function.