How do I validate the entries in a DataGrid to ensure that the user is filling the grid in correctly?

For example, if the user enters a price for 1 to 10 units as being £1.00 on the first row, but they accidentally enter a second price on the second row for 9 to 20 units as £0.90 - how do I catch it (i.e. it should be 11 - 20 units)?