Here goes a patch that solves this bug.

Created attachment 26414 [details] [review]
Use work and multiply it for resource cost

Is this really correct? Isn't it really the duration we want? You pay per time,
not per amount of effort.

I think you should get paid for the effort, otherwise, the whole concept of 
effort here must be clarified. I could do three tasks at the same time, 
dividing my effort (8h/day) for those three tasks, but of course the time to 
accomplish them is multiplied by three. if you get paid for the time and not 
for the effort you can multiply by three times the cost, which is of course 
wrong. The cost of doing the three tasks one after the other or doing them all 
together must remain the same, thus you get paid for the effort. 

What I mean is that the cost should be the time spent * assignment rate * cost,
so if you are allocated at 50%, and spend 1 hour and the cost rate is 10/h, then
the cost is 1h * 0.5 * 10/h = 5. We can't simply discard the assignment
percentage, can we?

In my company, when we talk about effort, we mean the man-time it takes to 
complete the task. Then if I work only on that task full time, it means that 
the calendar time spent on the task equates (numerically) the effort. So time 
= effort/assignment_rate. 
When instead I get to work on two tasks at the same time, then my 
assignment_rate could become 0.5 for each of the two tasks and thus the 
elapsed time it takes to complete the two tasks doubles. But the cost of the 
tasks remains constant because cost = effort * hourly_cost. 

That doesn't work if other people work on the task as well, then you don't do
all the effort yourself. The duration on the other hand is calculated from all
the assigned resources and the total effort. That's why I don't see the patch as
correct, even if there is a bug to start with.

I see it that when someone is 50% allocate to a task they are only working 50%
of the time i.e the rest of the time they are doing something else (coffee,
lunch, other tasks etc). From a billing point of view 50% of 100 EUR cost per
hour is 50 EUR cost per hour no matter if the task is fixed effort or fixed
duration.

Same with materials. If a material (e.g. a machine) is 50% allocated and e.g. it
costs EUR 250 per hour to run (in running costs) then the task cost is 125 EUR
per hour for that machine.  

If its unlikely that a machine would be re-usable for any other job then it
should be set as 100% utilised. 

If you got a mainframe that you use 25% of its resources, there's still 75% for
other projects to use. I'm thinking about simulation for example but this could
also be the same for some test benchs in industrial process.
From the top management, the cost attached to that use is still 25% of the total
cost. If other projects don't use it, that may go in other parts in the financial.
But that can't be generalised....
Maybe we should setup a check box depending on the wanted cost computation.

It would be nice to fix this for 0.12.

Hmmm, I think as I said in an email to planner-dev that with:

total += mrp_assignment_get_units (l->data) * priv->duration * cost /(3600.0 * 100);

but I will rethink more about it to be sure and create several samples to think
about them. Sorry for the first quick and wrong patch ;-)

Created attachment 27037 [details]
Test cases for cost computation

Attachment with the testcases sent by Xavier Ordoquy and
implemented by Lincoln Phipps:

http://lists.imendio.com/pipermail/planner-dev/2004-April/000421.html

The patch is commited, is this bug fixed then, or do we need more fixing?

Closing, please file new reports if there are specific bugs.