Acquiring new client is expensive, and when someone gives me a new client I usually reward them VERY well. It's hard to just put a percent on it, because each project has a different margin/profit but on a project with a 50% margin I will frequently give half of the first 90 days profit to whoever brought it in.
I wouldn't really think of it as 'percent commission'. Think of it as the 'cost of getting a new client'. How much would you pay to bring in a client worth 10k? 100k? How much is it worth to you? That is how much you should pay, because you want to encourage salespeople to perform well by paying them well.
If I could pay $2500 to get a client that will bring me 10k in profit, I will do that regardless of the percent. I'm happy if the sales person makes more.
The second part about the salesperson also being the project manager - watch out for that. Those skills are rarely found in the same person and it could get sticky. If they can do both, then they immediately become more valuable than you do, so they should get most of the profit