I've been looking into the process of starting up a business, and I came across an interesting fact about advertising. Start Your Own Business suggests devoting "2 to 5 percent of anticipated gross sales" to advertising (Lesonsky and Entrepreneur Media 493). And this is annually.
This might be a good guideline for indie/self-publishers who spend money on advertising. Actually, it is probably is helpful for any writer with a book out. I suppose if you use free versions of advertising, figuring out the equivalent value would work too. Two to five percent. That's Entrepreneur Media's guideline.
Cite: Lesonsky, Rieva, and Entrepreneur Media. Start your Own Business: The Only Start-Up Book You'll Ever Need. 3rd ed. Entrepreneur Press, 2004. Print.