my company pays £330 for me to be covered. but it doesn't cover any conditions for which i've had treatment/seen a dr in the last 3 yrs. if you took out private health insurance now, and have recently had a illness, it will most likely not be covered if it reoccurs again soon.
i've look at getting cover privately, and it was anything from £25 - £60/month. what these different companies cover varies widely.
also- please note private health insurance here in the UK generally works much differently than the US. in the UK, you still go to your GP for general care, and the private health insurance kicks in only under certain conditions (you need to see a specialist, you're quoted x number of weeks b/f you can see specialist, you're diagnoised with certain conditions, etc)