About 5-6 yrs ago we set up with PaidOnResults, I can't for the life of me remember the exact costs, I seem to have about £2k setup stuck in my mind, and I think they initially talked about having to "load" the account with £X to cover affiliate commissions before getting a credit account with them.
On another site we used the idevaffiliate software, so hard to build trust in your own solution though, even harder to get good affiliates. However if your sites are established you should have no problems doing this.
Edwin hit the nail on the head, I'd say 75% of our affiliate based sales came from 10% of the affiliates, even amongst that 10%, you'd have a few who stood above the rest.
A sliding scale is a good way to encourage them. I don't remember the exact numbers but we had like
1+ sale pcm = 3%
10+ sales = 4%
25+ sales = 5%
50+ sales = 7.5%
100+ sales 9%
The big guys come in and offer a "taste" so they'll smash the 100+ sales in a week and then ask what the best percentage you can offer is, worth having this in mind and how much you can cut into your slice.
Its also important to pick an affiliate network which suits your product, for example MoreNiche works well for propriety supplements and manufacturers with big commissions, PaidOnResults works better with main stream supplements (resellers) and adult toys/lingerie, and so on.
Edit: its about 3 yrs since I was involved in these, so could tell me very out of date and old software options.