The rounding is working fine, but the display of that number in your layout could be tweaked to match your formatting. Select the number field object on your layout, then in the Inspector go to the Data tab > Data Formatting section, select the number format chiclet, specify "Currency" from the "Format" pop-up, turn on "Fixed number of decimals" and show 2 decimals.
Note that this will round any precision less than 1 cent to the nearest cent in the display, but you should not consider this a substitute for your rounding calculation. This formatting on the layout object does not affect the stored data, which still needs to be correct to the precision of your transactions.
In layout mode select the field then in the inspector select data formatting. Pick Currency and the ensure that the checkbox to show the fixed number of decimals (2) is ticked.
Jeremy, you beat me to the punch. : )