At least you're right about the price
To buy, when they are available, they will work out at around £15 each when bought as a kit, but to replace they will be much less than (around £11) that as you'll not be replacing everything in the kit each time.
Unlike those the merchants keep (nearly all 20 spline) the kit has 10, 20, 24 & 28 splines. In addition it makes 1/2" cartridges with a seat depth of 21.5-25.5mm and for 3/4" cartridges (try finding them in merchants) it makes cartridges with a seat depth of 23-27mm. Oh, every cartridge is also universal opening both clockwise AND counterclockwise OR turn 360 degrees. TapMedic means a fix on a first visit over 90% of teh time for separate hot and cold cartridges.
As a point of interest, this kit is made in Sheffield not some village in China. Oh and when one eventually wears out, all you have to do is change the o-rings and ceramic disks (£2.50) then charge for a new valve.
How do I know this? Me, a working plumber designed it.
To be completely honest, you will make more money using the kit than you will changing taps