Copy data from one field to another

[jovuco]

How would I automatically copy the sum of data in a field of one form into another field on another form?

IE, sum of all the "price" fields on one form will equal the subtotal field on another form.

I know how to make the change on just the form, but it doesn't make the change it in the table. I need this change to be made in the table in the subtotal field.

thanks