Totalling number columns including commas (e.g. 1,234.56)

Hi

I'm having trouble working this one out. Its miles easier for me to format the source data for datatables with commas i.e. 1,234.56 instead of 1234.56.

However when I try and total I get NaN (not a number). Is there a way I can get the totals to work whilst still keeping the comma formatting?

Test Case here, showing the Retail column not totalling and the percentage column as well: http://live.datatables.net/igamim/2/edit#javascript,html,live

Many thanks!