Kit Consists of Concentrate (either of the Varietals used or neutral base) and varying amounts of juice which was flash pasturized. THE GREATER THE AMOUNT/QUALITY OF JUICE HIGHER THE PRICE. This is due to higher cost of juice/packaging and transportation. Concetrate is much cheaper than Flash pasturized juice . Much of it comes from Argentina or Chili.
Example a 7.5 litre kit will have about 3.0 liters + of concentrate and 4.5 liters or less of varietal juice
A 15 liter Kit will have 2.0 Liters +/- of Concentrate and up to 13 liters of varietal juice.
My exprience is the 15 + litre kit are much better than the 7.5 liter kits, and work out to being about $1.50 bottle more. I use the kits mostly for 2ns run wines fermented on the pomance (prssed skings/seeds ete) of 1st run wine from grapes.