I’m not interested in knowing how much to allocate towards online marketing, but how much to allocate between online marketing activities? Banner ads vs keywords vs content development, etc. Is there anything about online marketing activities that would make this analysis any different that were I doing it for off line activities?
Online, you have a much easier time measuring ROI.
There’s no simple answer to your question – each business is different. My suggestion is to try everything and measure the results. Make sure that you’re also split testing your ads to find the best combination of text, color, text size, etc.
As for how much money – it’s based on how much a new customer is worth to you (think lifetime relationship, not just the initial purchase). Then balance that with the cost to “acquire” that customer.