The best alternative plant is the Duboisia Hopwoodii or Australian pituri plant.
Some varieties are said to contain about as much nicotine as some tobacco plant cultivars at the lower end of the nicotine-bearing range. However the extraction of nicotine from Duboisia would cost more because most varieties contain unwanted alkaloids such as scopolamine and hyoscyamine, which would need to be removed from the final product
[see:
E-Cigarette Terminology -- Duboisia]
If Duboisia could be produced commercially, and the best cultivars chosen, then perhaps the nicotine could be extracted at a final cost of around ten times the cost of current liquid nicotine. This compares very well with either synthetic nicotine (which requires double the quantity since it only has half the bioactivity of organic nicotine) or vegetable-extracted nicotine, which both come in at around 100 to 1,000 times the cost of current nicotine supplies.
Note that many examples exist of statements that synthetic nicotine could be produced cheaply; and that the bioactive isomer could be easily produced; or that nicotine could be extracted from vegetables easily. None of these have ever produced any concrete result, whatever statements are made about them, and they can be discounted as unrealised and unachievable for any practical purposes.
None of these routes are viable, though, because:
1. It doesn't matter where the nic comes from, or what you call it, because the principal job of the regulators is to shut down vaping as far as they are legally able to. Therefore, whatever changes are made to the product, they will simply change the wording of the laws to suit. Whatever alterations are made, they will change the laws to stop it. They have absolutely no interest whatsoever in public health, they are there to protect tax revenue and pharmaceutical industry income.
There is a fundamental error commonly made about the role of government regulators in the smoking-related area: that they are somehow in place to protect the public, or willing to listen to science-based argument, or similar misconceptions. They are there to ban vaping as far as they are able, because it is a threat to immense revenues of at least a trillion dollars a year. They will do whatever they can to ban or restrict vaping and protect the status quo (by protecting smoking from any serious threats).
2. Liquid nicotine will always be available from China and India, where it all comes from (none comes from the US, it's just re-badged). When laws prohibit the legal purchase of products that people have every right to access, they find an alternative source. Liquid nicotine will always be available on the black market, although the quality is certain to be lower than current standards. That is the usual result of moving products from the legal to the illegal market since it removes all consumer protection.