The cost of keeping a cat over the lifetime of the cat assuming that you have pet insurance and buy decent quality cat food, use cat litter and provide to a good level will be in the order of $10,000 - $20,000 (USD). USD (American dollars) can be converted to the same figures in the UK as sterling (GPD) as things are cheaper in the USA and the UK is a rip off country. For other countries a normal currency conversion will be about correct.
Of course it depends on many factors. The biggest is what quality of care you provide. A lot of people try and steer clear of the vet! Too expensive. I am sure some people cause a cat's death because of this. That will obviously reduce the lifetime's cost because the life is shortened.
Also it depends of the cat's lifespan. Some say that purebred cats live shorter lives than random bred cats. In general this is true. The average for a purebred might be around 12-15 years while for a moggie it might be around 15-20 years, all things being equal.
It might be arguable that keeping a black moggie is the cheapest type of domestic cat to keep. Black cats are healthier than other cats and so are moggies because of genetic diversity. The gene that makes a cat black aids in disease prevention. That's a little tip!
It is a lot cheaper in India. See also pet care costs.