If you have to apply the same formula to all cells of a column you may use the "math" option for column configuring:
{ id:"diff", header: "Deficit/surplus", width: 180, math:"[$r,:2] - [$r,exp]"}.
Important! It's used without the "=" sign.
You may use "$r" to link to the current row id and "$c" for the current column field name.