Since the query is about sales training, you need to tell us what is the product or service that these salespersons sell? Why is their average education qualification? Do you well-customised sales process well in place so that given a chance to handle a sales call, ten salespersons also handle in the same fashion?
Though your question is on outsourcing selling Vs keeping the salespersons on the roll of your company, reply depends on the above information.
Training to salespersons will be required and on whose roll the salespersons are that does not matter. Training cost will not go away by outsourcing.
To reduce the sales cost, you need to generate records on why sale takes place and why it fails. Unless you have these knowledge assets, your cost of training cannot come down. Training is conducted to widen the knowledge base of the participants is a narrow view of the training. The real success of training lies in building the knowledge assets. While salespersons may come and go, these assets remain with the organisation. Therefore, I strongly recommend concentrating on building these assets.