I am relatively new at this, but what I found by my experience and what read is as follows:
An easy way to calculate the cost is based on the average liquid usage. On average, people vape about 2 ml per day (1.5-1.8 ml for me). say about 90 cents per ml. So, the daily cost is about $1.80.
Keep in mind that your "smoking" pattern changes. I vape more often than I used to smoke (more convenient), perhaps, in a shorter duration (no need to "finish" the e-cig unlike analog). In this sense, the average liquid usage would be a better way to estimate the cost than "pack."
The cost per ml can vary widely, especially if you buy high nicotine (36mg or 48mg) and dilute it with PG. PG cost only 2 cents / ml, at most, off-the-shelf. You could buy e-liquid directly from China, which would also affect the cost/ml. But about 90 cents/ml is what you can, without diluting, no hassle.
e-liquid: $1.80/day * 365 days/yr = $657
analog: $6/pack * 1 pack/day * 365 days/yr = $2,190
No contest!
You can also think it this way.
2 drops = 1 analog cig
20 drops = 1 ml
20 cigs = 2 drops * 20 = 40 drops = 2 ml
Which comes to 2 ml = 1 pack. Which means, again, $1.80 /"pack-equivalent" to $6.00 / analog pack.
You can also go by the nocotine content. People intake average about 1mg from smoking 1 cig. Analog cig has a lot more than 1mg, but most gets destroyed in, guess what, the fire. I heard only 1mg gets absorbed into body. On the other hand, in vaping about 98% of nic gets absorbed.
This gets little complicated because the varying nic content in e-liquid (0mg, 11mg, 18mg, 24mg, etc). Take 18mg liquid. It is 18mg / ml. So, 10 ml has 180 mg, which is equivalent to 180 cigs = 9 packs.
Hope this helps.