On top of the above-mentioned important points and questions mentioned by Tom Ramar, it is also important to consider the workload and investment required by the sales people. Are you selling a solution or a product ? Sales guys would need to spend a lot of time mastering it or is it an easy sell ? Will they have to buy or carry hardware with them, organize demo, workshops, showrooms ? Is there any form of exclusivity, like territorial ? Who are the typical customers ? individuals, large corporations etc...?
So it depends on many many factors... commission could range from a very low 3% (i.e. selling to multinationals multi-million $ contracts if the sales guys have good relationships) to 30% (retail sales, requiring traveling, demo, etc...).