Average cig is 0.8 - 1 mg of absorbed nicotine. The numbers posted for cigs are average absorbed, based on testing. A pack saying 20 mg is for the entire pack.
Eliquid nicotine is base value, not absorbed. From the limited test info available, work by people like Etter measuring cotinine levels, it looks like we absorb around 1/5 - 1/6 of the nicotine in eliquid. That means we absorb about 1 mg per ml of 6 mg/ml eliquid, about one full strength cigarette's worth. Absorption varies depending on experience and equipment, but it appears to be consistently a small fraction.
(The same is true for cigarettes - an unburned cig has a lot more than 1 mg of nicotine in it.)