It's called a finders fee and ranges from a very low of 5% to a very high of around 20%. Usually it is in the 8-12% range though. Really depends on how much work you do as a finder and what resources you expend. You need to negotiate this up front with your client (whether it be the person looking for funding or the person who has funds they want to invest - bigger fees come from people looking for funding) and have a signed contract. You can build other stipulations intot he contract other than a simpe finders fee (step up funding - 5% of the deal if you get them 1mm, 10% if you get them 10mm, some upfront costs to cover expenses, etc.)
Best of luck.