If you're just putting your initial startup plan together and you're wondering what a proper startup marketing budget should be, first recognize that if you haven't launched into the marketplace, you're still testing. You're not ready to budget at all.
Your initial budget should be highly segmented into small budgets that are used for testing, until you figure out what works.
A Few Thousand To Start
For example, if you're going to use Google Adwords, you should plan on segmenting a budget of $1,000 into a few smaller tests. Once you figure out what works (this could take months) then you can put together a more structured budget as you understand potential results.
What you want to avoid is just budgeting $10,000 per month with the assumption that you know how it will perform.
Therefore, consider a startup budget of a few thousand dollars, with the understanding that you really don't know what your budget should be until you've run your tests.
Test, Refine, Repeat
Don't get hung up on having all of the data and costs ready before you launch. No one has that kind of foresight. Just recognize that there will be a point in the not-too-distant future where you'll need to ramp up budget.
Until then, keep it small and focus on testing, not long term budgeting.