For the most part, digital analytics fail to allow the aspiring analyst to perform budgeting work. You know who converted and how they converted and where they came from. You don't know how much you "should" spend.
If you know nothing, go with a rule of thumb (but be sure to get the appropriate data so that you know "something").
Say you are trying to figure out an appropriate paid search budget for January. Here's what has happened each of the past five years:
The relationship can be depicted graphically ... and therefore, can be modeled.
Now that we have a relationship, we can estimate the optimal amount of spend.
Sit down with your CFO and go over your options ... you now have a relationship based on historical data that you can rely upon!!