What you can do is create a procedure that holds a starting value, checks to see if the value in a field is different from this starting value. If it is different, add it to a sum-value and change the starting value to the actual field value. Display the sum-value anywhere you want it.
Fire this procedure with a frequency that fits your needs.
The only thing is, if the value you want to add is the same as the previous, it wont detect a change so it wont be added