I have seen many methods of calculating salesperson commission.
I know a quick printer who pays between 4% and 6% of invoice to any sucker who comes to work for him. He usually starts the salesperson at 6% and then pulls downward. This is a commission against minimum guarantee of $15K a year (!). His people don't usually stick around for more than a year before they figure out that a shop doing $100K a month is going to divide $6K a month between its 3 salespeople who are cold calling hard to bring in customers who will become house accounts in a year.
I know a big printer who pays a salary or 8% of invoice, and says the salesperson who stays weans him/herself from salary within 12 months.
I know another who pays only salary (not even a bonus). Gets very high quality people. On the other hand, another shop pays a pretty high percentage of gross profit, and does very well in terms of whom it gets and what kind of work it gets.
All these shops use their salespeople as best they can and make money from it. Their salespeople (except for #1) are usually very happy and committed to their employers.
So... how much commission? It depends entirely on how you price things, who you have as customers and salespeople, and what the salesperson will accept.