I’ve always (since the dawn of time, or at least since 2004) been advocating the end of the concept of an online marketing budget for any online business where you can measure results.
The concept of a budget is faulty and precisely opposite to the approach you should take.
Scenario A - you spend a dollar on AdWords, and get 1.20 back.
Q: What should you do?
A: BUY AS MANY CLICKS AS YOU CAN
Scenario B - you spend a dollar on AdWords, and get 0.80 back.
Q: What should you do?
A: STOP BUYING
Notice how the concept of budget doesn’t really fit in?
In fact, it’s a pure cash-flow excercise.
If you want to make the lines a little fuzzier, go ahead and apply factors such as inventory management and alternative channels. Sure, you’ll need to time your marketing right if you’re selling momentarily finite inventory like, say, hotel rooms. And, you’ll have to consider if you can attract a specific customer or customer group even less expensively in another channel, etc, etc.
But the general idea is still - if it works, do as much of it as possible. If it doesn’t work, stop doing it.
If that’s not a blinding flash of the obvious, I do not know what is - and still, there are online marketing budgets.