I'd make sure you have a process in place in Salesforce to ensure there is a line in the sand regarding the difference between a lead and sale. I've seen issues come about if there is no set process in place, resulting in a grey area.

Also, I'd consider having a variable commission structure.
i.e.
$0-$50K = 10%
$50k-$100K = 12%
$100K - $200K = 14%
$200K - $300K = 16%
etc...
The more a sales a salesperson brings in, the more commission they should make.