Help with understanding this code and rewriting code

I found this code and I am able to use it in another program with no problem.  It gets me the total of the line items checked.

Now I need to change it but I haven't seen code written like this. Can someone help me read it?  I get the basics but I need help adding to it.

public static decimal GetCheckedSum(this DataGridView sender, string CheckedColumn, string AmountColumnName)
        {
            return
                (
                    from Rows in sender.Rows.Cast<DataGridViewRow>()
                    where Convert.ToBoolean(Rows.Cells[CheckedColumn].Value) == true
                    select Convert.ToDecimal(Rows.Cells[AmountColumnName].Value)
                ).Sum();
        }

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As you can see below if there is a BillAmt then there is nothing in the PaymentAmt Column.  When I run the above code I get an error because there is no number in the PaymentAmt column.

I have a grid that displays the amt of a bill or a payment.  If the user clicks on the checkbox I want to get the Total for the Bills and Payments.  Here is how I have the grid laid out.  If it is all messy I have attached a screen print to see.

CheckBox         ID                   BillAmt            PaymentAmt
                           45                    4,500.00                      
                           46                       200.00    
                           47                                               4,000.00
                           48                    2,000.00
                           49                                               5,000.00


I would like to add an IF Statement or something similar

 if (BillAmt == "")
{
    BillAmt = 0;
}

OR in the SELECT portion change it to this

select ISNULL(Convert.ToDecimal(Rows.Cells[AmountColumnName].Value), 0, Convert.ToDecimal(Rows.Cells[AmountColumnName].Value))

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