Any Networks do this? If so how much is a good amount to prepay an affiliate?
There are a ton of networks that prepay affiliates and even fund their media buys. It's on a case to case basis though so unless you have a track record and a good reputation you won't be prepaid.
Beware of prepays. I've been prepaid before then shaved to death. Not saying its always a bad thing to get a prepay, just make sure you trust the network you are getting it from.
It's a great option for the right affiliate/traffic.