IFf this is just for display purposes, then the easiest way is to handle the CellPainti8ng event for the DGV, check if it's the "difference column", and if it is, calculate it and show the value. I do something similar with couling DGV cells to indicate anomalous values:
Colouring DataGridView Cells According to their Content in WinForms[
^] - it's should be pretty easy to do what you wanted from that.
If you want it persisted back to your DB, then you would need to work on the data source, not the DGV .