Fairly certain that you cannot sum a calculate field - at least that was true at some point in time. That is/was because the values are not stored. The values should come from a table or query, which you'd then sum and do any calculations against that control but show in another control.
EDIT -To clarify, I mean that the calculated field is in the subform underlying query. I spent a few minutes looking at that and the tables but didn't come up with a way to fix it in that short time.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.