I wonder if someone can help me here:

My little software shop is creating packages for enterprises for years. So far it has been selling through word-of-mouth mostly. I am now at a stage where I want to go ahead more agressively with sales, so I am looking to get a sales rep in action.

My questions are:
In our industry, what is the percentage of $sales that should go to the sales rep ? My typical installation is around $15K per site (from $3K to $50K). Is there any kind of norms in the industry ? Any document about this on the net ? Is it better to pay only commissions or to give a base salary ?

Thanks for any input
Eric