What is a good ratio, in prices, between your bike and your lock so that it would be reasonable. Say if your bike is 200 GBP, a lock of 20 GBP might be too cheap (or just right!). So where do people draw the line?
Totally depends on the person and how much you're willing to lose. One of the main things you want to do is lock properly -- an expensive lock used poorly isn't very good. And you usually just have to be harder to break than the competition.
For U-locks, you can take out even higher quality ones relatively easy if there is a lot of room left in the lock by either using a jack or having a lot of free space to take out an angle grinder. And you may not want to steal a whole bike -- after a whole bike, going for the wheels or a seatpost+saddle are good options. So, if you bought something like a Kryptonite Evo Mini 5 and used it just for the rear wheel inside the rear triangle to a good bike rack (they'd steal your front wheel and seatpost; nobody's going to saw through the rim), you'd likely be better off than using a New York Lock through just the front wheel (they'd remove the front wheel and steal the rest of the bike), for example. Poor quality cables are easy to remove and same for thin chains (just use bolt cutters).
Your locking scheme should make it difficult for someone to remove the lock. For example, lock your bike to something that can't be easily removed or the bike lifted over it (e.g. a well placed and secured bike rack, not a flimsy sign). Make sure to use good quality locks which are non-trivial to cut. For U-locks, make sure to leave as little space as possible to get a pry bar in. You can use a small U-lock to just catch the rear wheel on a standard diamond frame to the lock and have very little space left and secure the rear wheel and frame. The front wheel can be caught with a cable or another U-lock. Seat can be done with a small pad lock+cable or something, or you can use security skewers. Just putting the U-lock through the frame isn't a great idea since someone can still easily walk away with wheels and seatpost+saddle. See these links from Kryptonite and Sheldon Brown for some more details. You may also want to use a heavy chain in lieu of one of these U-locks, since that requires different tools.
For what its worth, I use a Kryptonite New York Lock Long Shackle and a Fahgettaboutit (which is about 160 dollars of locks) on bikes worth under 300 dollars. But I use them properly, and I park on a college campus where, like most of them, bike theft is a problem (and I can't really afford losing my bikes). I use the good bike racks, and use the Fahgettaboutit to catch the rear wheel and the long shackle to catch the frame and front wheel (or reversed, depending on the rack). This leaves very little space to remove the rear wheel (and a hard lock to break), and makes it too much effort to remove the front wheel. However, if I was in the middle of some random small town or something, I might just be able to get away with a 10 dollar lock on the frame or even just a frame lock. You should be able to get a feel for whats necessary in your area by looking at well locked bikes in the area. And if you can bring your bike into your home, all the better (this means in a place you have complete control; not something like a communal garage where you still need to lock).
piggybacking on Criggie's comment...
What is a good ratio, in prices, between your bike and your lock so that it would be reasonable?
Well, sometimes the level of bicycle theft is rather unreasonable.
The amount of time you will leave the bike locked, and where you will leave it locked are also variables.
I get around Santiago de Chile with a $10 bike and a $10 lock. Combining this with a few more anti-theft techniques, I haven't had problems.
This is probably way overkill for a small town.
But it might not be enough for someone who wants to leave their bike locked overnight in London's worst neighborhood.
You can't think about locks based on their price.
There is an optimal lock: it's the smallest U-lock with disc detainer core that will fit around the bike and the thickest shackle for the smallest size. In most cases, the optimum is Kryptonite Evolution Mini 5.
The disc detainer core won't get any better by throwing more money at it. Picking it requires special tools and lots of skill and patience, so a bike thief most likely buys a battery powered angle grinder instead and waits until there's no one around to hear the loud sound of the angle grinder. The smaller the lock is, the more difficult is to orient the angle grinder such that it cuts through the shackle but does not damage the bike. So by throwing more money at the shackle, you'll get a larger shackle which is ... worse!
However, there are two cases when the "smallest U lock with disc detainer core" rule no longer applies.
One is fatbikes: they have so wide wheels that you can't lock the wheels using smallest U lock there is. However, you can probably lock the frame to a small fixed object but if there's no small fixed object around you're in trouble and even if there's a small fixed object it's better to lock the rear wheel around the object, instead of locking the bike frame around the object. So with fatbikes you'll have to buy a longer and more expensive lock. Being more expensive, and thus larger, it is worse. But in this case you can't buy the better and cheaper lock.
Second is folding bikes. There may be no reasonable way to carry a large U lock with a folding bike such that the bike folds without the lock hindering its folding. In this case, you'll have to buy something like Abus Bordo Lite Mini 6055. It isn't as secure as a decent U lock and the pin tumbler core is easy to be picked. But if you have a folding bike, usually you'll carry it with you and only leave it outside locked if you absolutely can't carry it for some reason. So this is a lock for very rare uses, perhaps 5% of the time you lock it and 95% of the time you carry the bike. Thus, the lock doesn't have to be any more secure. Actually the light weight of a lock with folding bikes is an advantage, not a disadvantage.