the difference is in the amount of ink in the cartridge. The more ink, the higher the price.
Low= a small amount of ink.
Standard= a normal amount of ink.
High= Lots of ink.
In theory, if you get a low cart with 1 unit of ink, a standard with 2 units, and a high with 3 units, the prices should not be £1, £2, £3, but more like £1, £1.75, £2.25. So you would be better off buying the larger carts. 3x as much ink, 2.25x the price.
But you have to know how much ink is in the cart, before you could actually work it out.
Not all carts actually say how much ink in cc's, (they just say, high, low or standard) so you could end up paying £10 for a low capacity, versus £20 for a high capacity, and the high capacity holds 1.5x as much ink, but costs 2x the price. Not a good deal.