Not all the websites have the same needs and not all the users have just one website, in the same way a service has to provide multiple tiers for satisfy all the needs and let the users pay only for what they need, here the first definition of tiers for this service!
For small websites that have just and handful of links, that do not need to be checked frequently, there is the Free tier, as the name say this is free and is going to be free for ever, even though is quite limited in the number of link that is possible to manage with it should be definitely enough for everyone that does have a website for not professional purposes.
For users that have a website with more links, or that need the links to be checked with guaranteed timing, there is the Basic tier, this allows any user that rely on the content of the website to be constantly correct, to have it, even receiving notifications when the third party content is changed, having in this way the possibility to review it.
If your business handle multiple websites, that may use the same link, you can exploit the Premium tier, this tier has the higher limit in number of links that can manage and also the ability to check the link availability and content more frequently guaranteeing a more promptly snapshot activation and notification of link offline.
These are the tiers defined today, but these tiers are not to be frozen forever, as soon as new features are added the tiers will gain some more capabilities, and as well as the services will be able to handle more cases, higher capabilities tiers will be added.Check the pricing page and join the service!!