Formulas In .net Form?

there is a datagridview, which will display Group Name in first column, Variables in second column.Users will populate it manually. will select a group name in first column, variable in second column. in similar, for some count of rows the group name remains similar, with different variables in second column.There may be different groups with lot of variables in second columns.our requirement is to insert a formula where ever its needed by the users.like getting subtotals at each level of groups.Also in case if they needed to have subtotal after two or three groups they must be able to define the row a formula to add those groups sum.