For us, we don't compete AT ALL with the other giant hosting companies. They're on a different level than we are. They have HUGE budgets (Hostgator pays out $300,000 per MONTH in affiliate fees) - that's not a level that we're even close to paying out.
We generally go with 10-20% payouts recurring monthly. We had tried doing the $10.00 payout, or the $25 payout but we found that if we offered the percentage payout our affiliates were happier as they knew the people they referred would be staying with us.
Once you have a solid foundation in web hosting, people recommend others without any thought of compensation. The 10% kickback is such a small price to pay, but it's SOMETHING (and that's exactly what we tell our customers too). We're not going to make anyone rich by referring our services, but you will make people happy with a reliable host
Coming up with the figures, you have to break down what it costs for each client to obtain. Clients usually run between $2-4 in marketing fees for each client (if you have a specific niche).
Much like in other threads here, the key is not to align yourself with the larger hosts, but to stand out. Why go with a small host rather than a big host - usually it's service and personability.
Most people will offer a larger comission for a larger customer (dedicated etc) and that's contingent on the fact that the customer will stay. Lets say you sell a server for $200/month. If you pay out $100 for a dedicated server referral, and your profit line is $40/month on a dedicated server, then you have to lock that customer for 2.5 months before making a dime. Now, if you're paying 10% comission ($20.00), you're making money right away, however your profit is now only $20/month for the life of the account. Which one makes more sense?
Places paying $50 for a $5/month account are usually shooting themselves in the foot, but when you have that big of a bankroll, and you're just looking to generate clients and resell other stuff to them, it works out.