The problem  seems to be you are attempting to add strings together to get a sum.
What I have done in the past is to assign  each of your input values  to a dummy
variables and to DIM each of these variables as Long. sum  the dummy variables
and assign the new value into your sum text  box. I run a macro to sum the data
whenever the input boxes are changed.

Also, I have an "update worksheet  button" so the sum is updated in the
database only when I'm ready.

I know there is has to be a cleaner way of doing this, and I look forward to
seeing more replies; but this will put  you on the right path.

: Now on my form  there is a field  which takes sum  of amounts as
: "=200+786+900+3" etc. I want to place  another text  field which
: should or would show  me the sum before placing the entered  data
: on worksheet.

Use Evaluate(). Draw a new form with two text boxes on it
and add this sub to the code. Type your expression (=200+786+900+3)
into TextBox1 and click the Tab key.

Private Sub TextBox2_Enter()
TextBox2 = Evaluate(TextBox1.Value)
' or TextBox2 = Evaluate(TextBox1.Text)
End Sub

I really appreciate your support  and help.
Automating office is real fun with VBA, and when there are guys like you
there helping and sharing its even better...