Calculate a total in a Table - Livecycle ES2

I have a table with 9 columns and 28 rows - I have created a further row at the bottom and I would like the  total that column

I have labelled each row 'cost' and the header row is labelled 'totalcost'

I have made the result cell a numeric field labelled total

I have tried formcalc but just cannot to get this to work but realise it should be relatively simply - can someone help please?

Thanks