Here is an interesting table published by Rand Fishkin on moz.com.
Have a look at the column to the right (I marked green). Basically, the table says: no matter what you budget is, anywhere from tiny to very high, the most important marketing channels for increasing sales stay the same:
- Conversion Rate Optimization
- Search Enginge Optimization
- Email Marketing
Considering how many other factors and variables Rand mentions, this seems to stand out as a nice heuristic or short cut.
Please note that sales still build on having established some level of brand awareness and trust.
Still, the top 3 stay the same, independent of budgets. Interesting.