hi Korhan,
you can do this by Inserting a Cross-tab on the Report Footer. have a look at the attachment...extract the contents and change the .txt extension to .rpt.
right click on the cross-tab and choose Cross-tab Expert. note that there are two Sums on the same field.
exit the cross-tab expert and right click on one of the summaries under YTY change and choose Format Field. under the Common tab look at the formula in the Display String dialogue. this is the formula that does all of the work. it uses the 2nd Sum value in the cross-tab expert and uses the Grid value functions that are available for the cross-tab. you can also go to the online help for crystal reports to learn more about these grid value functions.
you can go back to the cross-tab exerpt then the Customize Style tab to see all of the various formatting options that were used on the cross-tab.
cheers,
jamie