this sounds like a join issue.
The formula field is enforcing the join to the WORKORDERCOSTSUMMARY table which has no matching rows for closed orders.
In crystal the join is not automatically enforced. You have to set it to enforced or use a field from each table int he join (or end node in a chain) for it to become enforced.
You will need to set the join to an outer join.
See the answer and more discussion here
http://www.crystalreportsbook.com/forum/forum_posts.asp?TID=22852&PID=74819#74819