Tasty Vapors has what they call "Doublers", which are zero-nicotine e-liquids, any percentage of PG/VG you want, available in all the same flavors as their nicotine juices, that cost 1/4 as much. Using Doublers, I just order 1 oz. (30 ml) bottles of whatever flavors I want, and in the 48 mg strength. Their nicotine juices all all the same price - about $16/oz. - regardless of the nicotine level (from 12 to 48 mg) - so this is a no-brainer. You can then reduce the nicotine level to whatever you want, using the identical flavor of zero nicotine juice, so the flavor is not diminished.
i don't know if other vendors offer Doublers, but they should. Plenty of vendors will sell you zero-nicotine flavored juice, but at the same price as the nicotine juice, which makes little sense, economic or otherwise.
As for your calculations, I believe that much less nicotine is absorbed per puff with
vaping than with smoking, so you can't just multiply 30 cigs x .6 mg nicotine per cig = 1 ml of 18 mg juice. I think you'd have to vape 3-4 times that to get the same amount of nicotine into your system.
I think it might have to do with the absence of WTA (Whole Tobacco Alkaloids) from most juices. And interestingly, nicotine by itself is not nearly as addictive as nicotine ingested along with MAOI's (monoamine oxidase inhibitors) that are present in cigarette smoke. I suspect that MAOI's might be present in WTA's, and that a nicotine juice containing WTA's might deliver more nicotine to your system than the same juice without them, but I'm still working on nailing that down.