[Tom Bunch]

Just to be pedantic, I assume when you say "Cost" you mean effort. Cost is another matter.

There are two approaches to this. One is to realize that the task requires 4 "man days" to complete, estimate it that way, and let the applications calculate that two people working at 100% will complete that effort in 2 days. This is often the more useful and flexible approach, because it allows you to respond to changing demands and so forth.

The other is to go to the Task: Assignments inspector and tell the application that for this task, as assignments are added and removed, duration should remain fixed and the effort should be recalculated. See "Keep duration fixed".

-Tom