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.