There's no such thing as cost-effective and experience efficient. Such concept in RS does not exist. With the removal of the bot farms, herbs will cost what they cost. Unless you make Serums or clean herbs, you will lose money unless you obtain the herbs yourself.
It does if you can make more than the money difference in the time difference between the two methods.
I'm not sure why that's so hard to grasp.
I'll show an example.
Say you make 2k serums an hour.
You get 50 x 2k = 100k xp/hour with serums
You get 540k xp/hour with making extremes+overloads.
Serums are -9 gp/xp
Extremes+overloads are -36 gp/xp, I'm going to value overloads at 20k per potion (they are still useful even as a pure), this reduces the cost to -27 gp/xp.
93-99 is around 6 million xp
So serums will cost 55m and take 60 hours
Extremes+overloads will cost 162m and take 11 hours.
So by making overloads you save 49 hours at a cost of 107m
So if you can make more than 2.1m/hour, make overloads. You can easily make double that, even as a pure or a skiller. Assuming you rwt (you are on sythe after all), you can easily make 6x that with a minimum wage job.
In my mind saving almost fifty hours (that's an entire work week!) is worth it anyways.
Most other potions are around -20 gp/xp, and the same logic applies there.