The kit gives you about 3 washes then you need the Auto Dry Deionised Filters for £4.99, but you'll also want to buy the specific Flash Shampoo which is also £4.99, good for around 10 washes.
£20 for the first 3 washes, £10 for the next 10 washes.
I guess it all comes down to how often you wash your car, for me it's usually monthly so I don't mind the cost, and I hate doing it too!!!
This made me laugh (from their website):
Q: What will happen to my old bucket and drying towels?
A: To be honest, the employment outlook for towels is rather grim. Most will end up streaked with oil before being thrown out. But buckets often find new jobs quite easily. Some will work as paint buckets or mop buckets. Most become holders of things.