Where you are leaving it and for how long is crucial information as @vickster has said. We have expensive off road touring bikes/expedition bikes and where I am happy to leave them and for how long is a completely different story to my road bike which was purchased for the sole purpose of being able to leave it at college (think £299) and even that is left with a £80 lock on it. (All my locks match, so I don't have to worry about which one I pick up in the morning when unlocking the road bike from the touring bikes.)
So if it is being left out side, out of sight for any length of time, you are going to be wanting 2 lots of something really substantial - like motorbike substantial.
We store our touring bikes outside, under the kitchen window. They are locked onto the 4 bike tow bar mounted bike rack, and then locked again with our locks to each other. It's secure for where we are, rural, we could easily leave them unlocked here. No-one even knows the bikes are there, it's totally out of sight and you can't actually get the bike rack out with rotating it onto vertical, so I have no issues with the bikes effectively being on the rack "ready to nick". It would take 4 people minimum to get it out of where it is and that just would not happen where we are - and that is the critical bit "where we are".
For someone in a town/city wanting to leave a bike, it is another matter entirely and the only advice that is practical, is to make your bike look a less enticing 'victim' than another bike. As vickster says - miniumum 10% of the value of the bike should be spent on locks, though I have to say the idea of using £250 of locks per touring bike is frankly ridiculous.
If you have insurance, then you need to check with them and what they require. Most stated "sold secure gold" and some even go as far as "sold secure gold & cat 15".
One bit of advise given to me years and years ago was to use 2 different styles of locks to lock you bike. Apparently, though this may have been then rather than now, most bike thieves only come prepared to cut 1 type of lock (spiral cable or D lock) and bikes with 2 locks are less attractive than bikes with 1 lock.
Also make sure that your D lock is not too large, car jacks can be used to easily remove them if there is space to get the jack into. My brother has legally used this method to recover a motorbike after an RTA where the rider had locked it after the crash and then been taken to hospital. He rang his supervisor to ask who he was meant to recover it when it was locked and was told this method and was horrified at how easy it was.