@Yaya -- it variously enormously by industry. Ad rep firms will take anywhere from 25-60% of ad revenue from a small publisher (in essence, that's what Google does for all the media they sell on other publishers' properties). On big ERP deployments, referrer sales reps may just take a few points.
What drives down the commission:
a) the margin in the industry
b) how much marketing you are doing to build awareness?
c) whether you are handing off warm leads to the sales rep
d) what sales support you provide
e) deal value
f) time/effort to close
g) how difficult it is to access/close those clients
h) what resources your friend needs to invest to close
If your friend doesn't have some secret sauce, industry experience, amazing network (i.e. some form of unfair advantage) then it doesn't really matter that they are your friend; their cost of closing deals will be higher than a specialist who knows the business well. If your friend has an unfair advantage then you should jump on them and lock up their commitment.
While declining payouts might look nice, there are lots of downsides for both parties that you need to consider:
a) there's real maintenance cost involved to paying reps over the long-term
b) most deals start small and grow over time and that increased value is usually attributable to account management (+ evolving product) over sales -- you will pay too much if they get a cut of everything from the account
c) it can get real contentious on high-value accounts
d) at the start, just having customers matters most so someone needs to get customers....
e) most sales reps optimize for cash-in-hand vs residuals (especially when dealing with startups)
At this early stage, get the deal done with your friend at a mutually agreeable price that doesn't come with long-term maintenance. Depending on time to close accounts, you'll know the unit economics after X months and you should write into the agreement a review process at X months to reevaluate the relationship (and pricing).