Calculated data item is not working as expected

[Oswald]

probalem: I am creating a calculated dataitem and want to show the total of the column but somehow its not working as expected.
I am using a relational databased

description: I have 3 data items productline, cost and leftover
I have created a new calculated column cost_numeric as (cost - leftover / cost) and the output is working as expected I do see a cost_numeric value for each productline
now I need to add to show up the average value of cost_numeric, for some reason the cost_numeric is showing as 0. I have tried tweek the aggregation functions but no luck. I thought of create 2 data items like total(cost_numeric for report) / count(productline)
But total(cost_numeric) itself is not working I still see 0 as report output.....I have also tried derived query method but no luck

Any help on this is much appreciated Thanks!