This page provides a little guidance as to the typical security products used for items of certain values. It is necessarily somewhat vague as different rules apply to different situations, such as a bicycle in a secure shed may warrant less security than if the same bicycle was out in the open, and more security than if it was in a garage.
Police guidelines suggest that you should consider spending around 10-15% of the value of the item(s) on their security, but this can break down at relatively low, and at relatively high, values. It should give you an idea, but you should also include the wider situation in your neighbourhood and how desirable the items may be to thieves.
It is also a good idea to consider the inconvenience of an item being stolen, as well as the actual value of that item. E.g. if you can't get to work or college because you've lost your main means of transport, that may be justify a higher expenditure on its security than the pure value of the item might normally suggest.
It is also important to keep in mind the local situation and, especially, if there has been a previous theft. We normally follow the guidance summarised here and then go up a level, or even go up two levels, if there has been a previous theft. Once a thief has managed a successful theft, there is sadly a good chance he will return and it's important that you then regain control of the situation, if at all possible. Otherwise, you can end up playing catch-up with thieves indefinitely.
Click on any of the following links to view or expand the explanation.