In essence, e-cigs combine a 'filter tip' and a battery.  The tip is either a cartomiser (pre-filled with nicotine), or an atomiser which just does the mechanics, but has no nicotine in it.  You have to add the nicotine by way of a 'tank' or a cartridge.  Alternatively, some tanks, particularly on larger e-cigs, combine the nicotine container (tank) and the atomiser, into something called a 'clearomiser'.  These are the best choice in my view.

Most suppliers use cartomisers.  They cost about £1.50 - £2 each, and each one will give you between 8 and 12 cigarettes' worth in my experience (rather less than the claims made).  The reason that the claims rarely match the experience is that the claims are based on a number of puffs per cartomiser.  But a puff is usually measured at 2 seconds, whereas most smoker 'drag' longer than that.  So, whilst a cartomiser may yield 'n' two second puffs, it will only, of course, yield 0.5 'n' 4 second puffs.  

Next - liquid.  Again, the prices vary, but a reasonable average is about £6 for a 10Ml bottle.  Now, there is some argument about how much nicotine from an e-cig gets into your blood, compared to an 'analogue' cigarette (a normal cigarette), but I am going to assume that this argument applies equally to both cartomisers and liquid options.

A 10 Ml bottle of e-liquid, contains about 200 drops, and each drop is the equivalent of about 1 cigarette, if you use the right liquid for the kind of smoker you were - in other words, you would not use a 6Mg (low) strength liquid, if you smoked Marlboro Red (a 24 Mg liquid is probably more appropriate).  [see Which Strength of Cartomiser or Eliquid is right for me?] Therefore, I am going to assume, as a guide, that each 10Ml bottle contains the equivalent nicotine of 200 cigarettes.  I will further assume (for the sake of calculation) that each cartomiser contains the equivalent of 20 cigarettes-worth of nicotine, again, if you are chosing the 'correct' strength of cartomiser.  So, for these purposes, a 10Ml bottle equates to 10 cartomisers.  

Now, a 10Ml bottle costs about £6 and 10 cartomisers cost £15 or more.  10 packs of cigarettes will cost you £70 or more (as you already know!)

Whilst both offer significant savings over normal cigarettes, liquid vaping is, in my opinion, well under half the price of 'cartomising'.

So, that's easy then.  Is it?  Are you the kind of person (like me) who leaves everything til the last minute?  Can you see yourself rushing out of the door, late for an appointment, and, bugger!  You've forgotten to fill your cartomisers or tanks!  Or maybe you just can't be bothered with liquid - maybe it's just too much of a drag (sorry).

My recommendation is this.  Refill if you like, but order cartomisers as well that you keep in your bag/case/pocket...This way you get the cheaper option of liquid, and retain the 'convenience option' of cartomisers.