This kind of insurance is generally based on your income though the amount you insure is entirely up to you.
As a general rule, people tend to insure somewhere between 70% and 80% of their income.
There is also a choice of "wait periods". i.e. after x amount of days, the insurance company will start to pay out.
Remember, this type of insurance is taken out for long-term sickness so if you have the flu and are bed-ridden for a week, it's not for that kind of scenario.
You can choose the wait period - the longer the period, the lower the premiums as you're taking on more of the risk (kind of like the deductible/franchise on health insurance).
You can generally choose from 15, 30, 60 or 90 days.
It sounds like some people here have the 30 day option.
It can appear rather expensive, true, but then if you're independent, how long could you last without working? That's the thing you need to ask yourself.
Nearly all the insurance providers do this kind of insurance. It's worth shopping around.
The premiums may vary depending on the sector of activity, your age, the length of wait period, the amount you wish to insure etc. etc.
Hope this helps