Textbox Overflow Error

Dividenar

The overflow error arises since your code says:

If txtMarkup.Value = "" Then txtMarkup.Value = 0

so your following line:

txtNegAmt.Value = (CDbl(txtTOTALcst.Value) / CDbl(txtMarkup.Value) * 100

is a "divide by zero" calculation which overflows the maximum value of a Double variable or any variable in fact, since it's infinity! 

I also think your logic is wrong in that the first line:

'If Not txtTOTALcst.Value = "" Then txtTOTALcst.Value = 0

since it always sets  txtTOTALcst to zero (and furthermore 0/0 calculations are indeterminate!).

In the attached revised file, I've:

1. commented out/ replaced that line and
2. corrected your Negotiation $ calculation and
3. rounded up the resultant Negotiaton value to 2 decimal places for currency (though you may need first to change the end of that line from the form "###.00" to "###,00" for your locale). You might want to do similar for total cost.

See changes in bold:

Private Sub txtTOTALcst_Change()
    'If Not txtTOTALcst.Value = "" Then txtTOTALcst.Value = 0
    If txtTOTALcst.Value = "" Then txtTOTALcst.Value = 0
    ' make this a default value
    If txtMarkup.Value = "" Then txtMarkup.Value = 10
    'txtNegAmt.Value = (CDbl(txtTOTALcst.Value) / CDbl(txtMarkup.Value) * 100)
    txtNegAmt.Value = (CDbl(txtTOTALcst.Value) * CDbl(txtMarkup.Value)) / 100
    'format that to 2 decimal places (change decimal marker if needed)
    txtNegAmt.Value = Format(WorksheetFunction.RoundUp(txtNegAmt.Value, 2), "###.00")
End Sub

Private Sub txtMarkup_Change()
    'If Not txtTOTALcst.Value = "" Then txtTOTALcst.Value = 0
    If txtTOTALcst.Value = "" Then txtTOTALcst.Value = 0
    ' make this a default value
    If txtMarkup.Value = "" Then txtMarkup.Value = 10
    txtNegAmt.Value = (CDbl(txtTOTALcst.Value) * CDbl(txtMarkup.Value)) / 100
    'format that to 2 decimal places (change decimal marker if needed)
    txtNegAmt.Value = Format(WorksheetFunction.RoundUp(txtNegAmt.Value, 2), "###.00")
    sh.Cells(iRow + 1, 11) = txtMarkup.Value
End Sub

Alternatively, you might use MROUND to round to the nearest 5 cents (say, =0.05), using:

txtNegAmt.Value = Format(WorksheetFunction.MRound(txtNegAmt.Value, 0.05), "###.00")

(again adjusting the decimal separator as necessary).

Hope this works for you (now you can easily add cost and negotiation value to get Selling Price).