Is it really necessary per business process that the user enters the price manually?
Any possibility to get the prices from condition records without an option to modify?
If yes, then your problem would be really easy to solve.
If not - then I would try to understand why the user does that - is is because of insufficient training, or because of incorrect cost values in the order or he is trying to misuse the system. If it is the latter - is his manager aware of the situation? Maybe the problem can be resolved internally and no system settings will be needed.
If it is not possible to reason with the business users and if you absolutely need additional controls, then I can see two possible options for now - play with the functionality for customer expected price or with user statuses.
With user statuses you can try to create a workflow, which will set the status to approved for positive margin and to set to not approved for negative. Then you send the work item to his manager for approval. After a month or so of dealing with approve/reject the manager will surely deal with his subordinates 