Validating Data in a gridview

Question

I have a grid as follows.

Course         Fee Amount(dynamic textbox)
------         ----------
  A1             15000 
  A2             5000

I have a textbox(outside the grid) txtTotalFeeAmount with value say, 30000. My requirement is that the Fee AMount of each A1 and A2 should be lesser than or equal to 30000. Also A1+A2 should be lesser than or equal to 30000. How do I do this?

EDIT

I tried the following, it doesn't work.

For Each gvRow As GridViewRow In gvCourseFeesDetails.Rows
        Dim lblFeeHeadAmount As System.Web.UI.WebControls.Label = gvRow.FindControl("lblFeeHeadAmount")
        'Dim lblTotalFeeAmount As System.Web.UI.WebControls.Label = gvRow.FindControl("lblTotalFeeAmount")

        totalFeeHead = totalFeeHead + Val(lblFeeHeadAmount.Text)

    Next

    nIndividualFeeHead = Val(txtFeeHeadAmount.Text)
    TotalAmount = totalFeeHead + Val(txtFeeHeadAmount.Text)

    If TotalAmount > Val(txtFeeAmount.Text) Then
        Dim script As String = String.Format("ValidateAmount()")
        ScriptManager.RegisterClientScriptBlock(Me, GetType(Page), UniqueID, script, True)
        Exit Sub
    End If

My javascript function:

function ValidateAmount()
    {
        alert('Sum of Fee Head Amount should not exceed the total fee amount entered...!');
        return false;
    }