It's the way they built their prices, history. They did not base their prices on how much liquid goes into a cartridge; or at most that was only one of the factors (and of course that decision is logical: whether pipe, e-cig or mini: each cartridge needs a core, flilling, a mouthpiece, a lid of some kind; and those will be the bulk of the cost for the manufacturer). This works out in a price-system where you pay relatively almost the same for any cartridge. While the mini holds half or even less then half of the e-cig, the price is far from half. And the pipe-cartridges, holding 2 to 2.5 times the amount of liquid as you can find in the e-cig (which is around 1 ml, the e-cig classic; the pipe is 2 to 2.5 ml) are not at all 2 to 2.5 times the price of the e-cig.
As to the price compared to the liquid: until short ago, it was cheaper to buy the bottles with liquid. A little cheaper if you bought the smallest bottles of 8 ml (they didn't have 4 ml then); a lot cheaper if you bought bottles of 30 or 50 ml.
And then came the olympics... e-cig wasn't allowed to send out bottles of 30 or 50 ml anymore due to security-reasons, so only the very slightly cheaper 8 ml were left to send out. Then those got banned from express-routes too, and became a pain for e-cig to get out of the country. To still keep selling at a reasonable rate, they then lowered the price of their cartridges: the 'shape' that they are still allowed to send by express (guess that has to do with the fact that governments and airlines don't think a cartridge can be a bomb; and they do think a bottle can be; so bottles are a risk, and a bigger risk around any worldwide event). Anyhow, by lowering the cartridge prices temporarily, the liquid-amount in the biggest cartridges went lower then the price of a same quantity of liquid in an 8ml bottle.
Be prepared for cartridges going back to their former prices once the olympics are done and e-cig is free to send out all bottles by express again though. Then the pipe-cartridges won't be cheaper then the liquid in bottles anymore; so who would thén still put the work into squeezing those out instead of just buying a bottle. But as long as only cartridges can go through the express-route; and the liquid in the largest of them is actualy even cheaper then in bottles - that seems the best route to go for now..
