Seems the trick to get ~2-3x more traffic is to purchase bulk ads of value multiple of 1 or 10.
Here are the results with 10x $10 ads:
For comparison, 10x $5 ads did way worse:
To prove/disprove this theory I've repeated it 3 times with both $5 and $10 worth packs and got the same results.
Though, don't ask me why it is like this. Just enjoy the extra traffic.
Edited: Edit by SolidSnake: Fixed image links