When the market wouldn't bear the price that the mercs want -for getting shot at- how many of them do you really think will stay mercs.
You talk of todays security firms. I told you why they are so big and why there are so many of them; and that is because the individuals who just get hired start making $100k and over. THis is a very good incentive to get people to do that job. If the pay is $20k; I highly doubt that as many people would be as willing to get shot at, wouldn't you?
You assume that there will remain the high level of security companies and security specialists in a voluntaryist system. I doubt your premise because, as I've stated, who will be willing to get shot at for $20k? Not many, and especially not the people who are currently employed as security professionals.