We use a CPC-H, that codes part time, to code for the facilities we manage and are charges $3 - $3.50 per operative report coded. Other larger companies will charge $5 - $14 per operative report based on specialty. Ortho and spine are generally in the mid to high level of those charges. If I were going to do it, I would ask myself on average how many cases could I code per hour and then multiple that by the fee per operative report and see if that is good for you. An example is if you charge $5.00 an operative and code 10 reports an hour, you would be making $50.00 per hour.
I think it would be a great idea, but you need to consider a lot more then what to charge. Would you be a part time employee or a contracted service. If you are a contracted service, don't forget your expenses. Would you need to buy coding books, software, how are reports being sent to you and how are you sending the coding back to them. You would also need to bill them and pay taxes on the money received. Would you need liability insurance? What is the volume of cases? How would they communicate with you during normal working hours? If it is a small volume, I wouldn't worry too much about it and go for it.