Basically, this depends on your strategy.
As a general rule, you need a lot more money to get started with pay-per-click than you do with search engine optimization.
We recommend that anyone wishing to get started with pay-per-click has at LEAST $1000 up their sleeve. Be prepared to burn through a bit of cash before you hit on a profitable keyword/landing page combination.
On the other hand, if you're starting with SEO, then affiliate marketing can be as cheap as you want it to be! At a minimum, you need about $10/month for website hosting, and $10/year for a domain name.
However, if you can afford to spend a bit more, then it certainly makes life easier because you can pay people to do things for you, like writing articles and designing your website. (Check out the free lessons in Content Creation for more about that.)
A small budget of $20-$50 is fine, but you'll need to work harder. If you can afford to spend $500 or so, then your life will be that much easier! Remember, everybody has different skills, learning ability and budgets, so don't feel you necessarily have to follow the same route as your favorite guru - just go with what you can afford.