Hi Teddy,
I don't like very much the approach of focusing in "how much" before having a very good idea about "what", "to whom", "how", and "why".
The way I like doing this is:
1) I think about the target market (who do I want to send the message to?)
2) I think about ways to reach the target marketing: the media they use, the language they like, the keywords they use, etc.
Once I've defined the target public and the product, I can think about message options (what to say and how to say it). Each message option has its own cost and limitations and that is the point where I start thinking about budget.
As you see, each marketing action has its own cost and it doesn't have much to do with your income. This situation is even worse at the very beginning when your income is expected to be lower.