They range from around £7.99 upwards to £80.00 depending on what type you get.
There are little simple ones which have cartridges and can be recharged, they look like cigs and smoke vaguely like cigs. They tend to be about £8 for 5 refills, which equals 200 cigs.
I have one which you put a liquid in which is vapourised by a little heating element, and it cost me £31 in total with 2 bottles of liquid in different flavours. It came with a charger included. Apparently the element needs to be replaced every month or so, but I have heard they can last several. The bottles of liquid are around £4.