Quick 
subjective calculations 
I smoked three packs a day.
I vape about 5ml a day. 
The tax on three packs a day in Ohio would be about (just to make it easy) $9. Cost of cigs per above $2.60, so $3/pack for tax (total cost per pack @$5.60/pack) 
A 10ml bottle of eliquid (approx., ymmv, etc.) $5. 
So... two days of vaping (10ml) vs. 2 days of smoking (6 packs) would be $18 tax on the smokes. 
10ml bottle of eliquid for $5 plus $18 for "equivalent" taxes = $23 for a 
taxed 10ml bottle of eliquid. ymmv greatly 
Understand, there are many 5ml vapers that only smoked 1 pack a day - hence the emphasis on 'subjective'. Plus the gov't is going to want 'back taxes' on all the reduced sales in cigarettes caused by ecigarettes