[danielf]

Further to what Dai said: roughly speaking, E-cigs come in two 'flavours'. One is the 'looks like a fag' variant with the blue tip that lights up. These tend to come with prefilled cartridges which sell at about £2 a pop (~1ml). The other variant is the one I and Dai linked to. The jacovapour that Rizzy mentions looks like a rebranded ego (that's the one Dai mentions). These systems generally come with better batteries, but are also intended to be refilled from a dropper bottle, meaning you can buy the liquid in bulk which works out a lot cheaper.

The amount of liquid you will use depends mostly on the nicotine strength you buy. The liquid comes in varying strengths and you will go through more if you buy lower strength. My understanding from reading forums is that around 2ml a day is around average. At the prices Dai mentions that's about £1 a day, which is considerably less than regular smokes. A nicotine strength of 18mg/ml seems the average starting level for a 20 a day smoker. Many people gradually lower the nicotine strength once they kicked regular smokes. I lowered it quite aggressively to 0 over 6 weeks. Other people are happy to stay at higher nicotine levels because they enjoy consuming nicotine in what is perhaps not a risk-free manner, but ultimately far safer manner than through the burning of tobacco.

Words of warning:
1. Nicotine is a poison, and a strong one at that. A 5 ml bottle of liquid @18mg/ml will kill an adult man if it is drunk (likewise, three cigarettes would kill you if ate them). You will only absorb a fraction of the nicotine when used in an e-cigarette. Nevertheless, this is dangerous stuff, and should be kept out of the hands of children.
2. This market is unregulated. 'E-liquid' is mixed by people in garages and back rooms, and while there's little reason to assume it doesn't contain what people say it does, there's no guarantee either. Then again, there's little control over the harmful stuff that goes into cigarettes.
3. This may not last. Several countries in the world have banned ecigs (at least the nicotine containing variant), on the grounds that there's no quality control. The reasoning appears to be that as a nicotine delivery system it should be classed as a medicine and licensed as such (which would make them considerably more expensive). The pharmaceutical industry which manufactures standard NRT (nicotine replacement therapy) would love this, so they can keep selling us gum and lozenges etc at inflated prices because they are medicines.