The old adage "you get what you pay for" comes to mind at this point. If the affiliate networks are out of your price range or you want to manage your own in house affiliate program, there are two software packages that come to mind - iDevAffiliate and JAM. Depending on what you mean by cheap, both may seem expensive, but they come with support if anything should go astray and they both have a ton of features for both you and your affiliates.
I currently use iDevAffiliate for a couple of my affiliate programs and it works well. I especially like that it has a PayPal mass payment feature, but it's not perfect. There are some areas which could use improvement and the admin panel isn't as user friendly as it could be. But, for the price, it's worth it.
JAM looks like one great piece of software. They even offer a free trial download, so it doesn't hurt to test it out. It has a lot of graphs/charts that iDev doesn't have and it seems as though the admin panel is a bit more user friendly. As for the feature-set/functionality, I cannot comment on because I haven't used it.
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