Another option is to use "dosed" portions of nicotine to spike your 0 mg juice:
Concentrated Nicotine Solution Packets | Liquid Nicotine
They have a handy on line calculator that you may find useful.
I can appreciate you suggesting this method. I'm sure for small batches it works out economically. That noted I just sat and did some calculations.
$13.35 / 5 = $2.67 This means each unit is $2.67.
Now, let me say I want to do a 1 L batch of 18 mg nicotine base juice in max VG. Even using the
40% packs I still need to use 24 of them.
24 * $2.67 = $64.08 That's $64.08 for only the nicotine.
Now, let me say I get my
VG for $7.00.
$64.08 + $7.00 = $71.08 That's my total cost for 1 L of base juice using your suggested method.
I am not going to say my method is better, worse, or in between. For myself it works out more economically. I simply go to Heartland Vapes and pick up a
1 L jug of nicotine solution for about $30.00.
Granted at this point I'm not too sure of what the taxes on the nicotine will be exactly. I do not believe anyone actually is too aware of what the rate of taxation is going to be as of yet. If I'm in error my apologies.
Well, I just desired posting to present a different
possible method.
Another possible method is buying what is called 100% nicotine in solution and using it by simple formula to get the strength you need for your target batch.
(Target * Strength ) / 100% = Nicotine required
Let me present a quick example.
You've a 120 ml bottle of 0 mg juice and want 3 mg strength juice.
First you multiply your target by your strength: 120 * 3 = 360 .
Next you divide that by your solution strength: 360 / 100 = 3.6.
Third you apply 3.6 ml of your nicotine solution to your target 120 ml bottle of 0 mg juice.
There are plenty of reputable vendors online offering 100% nicotine. What you need to realize is that it is not actual 100% nicotine but rather 10% nicotine in solution so as to be safe for use by laymen. Actual pure nicotine would be 1,000% nicotine. At least this is how it got explained to me many moon ago.
The formula above works no matter your choice of strengths, target sizes.
(Target * Strength) / Concentrate = Nicotine Concentrate required to go into Target == Finished Product at your desired Strength.
You might want to look up Heartland Vapes, Wizard labs and some other nicotine vendors. I think Ecig Express still sells nicotine solution as well. My Vapor Store sells it. I could be all day running down a listing but won't.

What you may need to contend with is to be sure you look for what is now called "free base" nicotine.