What I would suggest is to make it easier. Without extending standard screen. Dunno whether it would be ok for you but I am thinking about :
1. Use badi MB_DOCUMENT_BADI correct method which is fired after creation in MB31. Here you could for example calculate your price dynamically and store it somewhere. In this case user would not be able to change price. I mean system would calculate it.
2. You could create your own transaction ( SE93 ) for report. Where in report we could select orders from MB31 and set price manually. This option is much easier then the one you are thinking about. Better to spend some time to think how to convince your client for that case instead changing standart SAP.
