Here in the UK we are now only able to buy nicotine shots, I usually get mine from Darkstar Vapour for £16 (around £20 shipped) for 180ml of 18mg Nicotine, it’s quite expensive.
For example, to make 100ml of 3mg e-liquid, I have to use 16.67ml, thats just over 1 and a half bottles, when you do the math you can see that I can make around 10 100ml batches, sure 1 litre seems like a lot but when you compare that to a bottle of 72mg nicotine which can produce quadruple that amount and was a similar price when available, you can see what I mean.
I have to admit though that the 10ml bottles are slightly overfilled, I can only assume they do this because these little bottles like to hold a ml or so back or maybe its just by accident.
Quick post before I get off to work - I have been wondering how literal the TPD actually is since the nic strength is expressed in mg/ml I wonder if nic salt powder which is mg/g would be regulated the same way. Chemnovatic sell b2b nic salt powder at 500+ mg/g and a few labs in China sell it at 300+ mg/g.