I use to sell this type of cover when i was a mortgage adviser. You can get several different kinds, accident and sickness (1 year policy), income protection and critical illness cover. As previous posters mentioned it depends on a range of things, age, sex, health, smoker etc.
One option is to go for accident and illness cover with a 4 week deferred period and get an income protection cover with 52 week deferred period to kick in after the accident and illness cover runs out. That way the premiums should be lower and short illnesses and accidents will be covered by the cheaper one year policy leaving the income protection insurance for long term claims from either accident or illness. I seem to remember critical illness cover was the more expensive cover. You should also make sure you have sufficient life cover to pay off mortgage and maybe a lump sum or family income cover if you are married or have a family. Life cover and accident and illness cover are relatively cheap.
Best to seek professional advice from an independent adviser, as i'm no longer licensed and a maybe a little bit out of date. When i last looked at it for myself, i'm 47 non smoker and married with kids i got a quote for around £33 per month for accident and illness and £65 per month for accident and illness, but as with all things prices can fluctuate widely over time and in different circumstances.