How can we implement a Billing cutoff service in settings?

I'm not so sure how useful this would be. In order to achieve something like you suggest with your example, the $5.00 Linode would have to be shut down at the precise point of the 1TB outbound transfer limit, or slightly before that point. In addition to this, if you spun up another Linode, you would go over the limit for the month without deleting one of the Linode's before the limit would be reached. You would also be disallowed from using any backup services and the like, if you planned to have the Linode on for the entire month. Something like this seems as though it could get very complicated, very quickly, for both Linode and its customers.

Hmm, great point, maybe apart from backup, all services which use network(as the only uncontrollable factor for cost estimations) should be stopped , offcourse if customer chooses to. Like there can be a price point after which services will be put on halt and customer can resume from there, as per their requirements.

This would only work for services that have a constant static cost per month, and if you wanted to limit yourself with some kind of billing cut off service, you might have to allow or disallow certain things. Putting services on halt for the rest of the month, without deleting them, will still cost you money. Shutting a Linode down without using that Linode still reserves the resources for you, and still costs you money. Having a detached block storage volume reserves resources and costs money, an object storage bucket costs money if it's unused due to resources being available for your use, etc. If any of these services was going to go over the limit, to prevent it, they would have to be deleted, and if you wanted that to be automated, you would have to choose before hand, what would go first. Deleting a block storage volume, a Linode, disallowing object storage buckets to remove the service charge, will all result in data loss. If you have some kind of production services in place, you wouldn't want to lose data. And I would personally rather go over the limit and keep services around, than set up some limit and have things removed on me without knowing when a limit might be hit. I still think this would get complicated quickly.

Let me add some points to uncomplicate things here..
As per service a linode is charged hourly, but capping is there that you will not be charged more than $5, deleting is not a concern here as we know the upper limit, Data service is unpredictable, doesnot have a cap, so leaving all services as it is, can we put a cap on only dataservice for customers who may benefit from it and have an option to toggle it on/off? Like 1 TB is limit for linode with $5/month, then maybe we want to have a cap on data service that after bill reaches $10, the linode will be shut down to prevent further charges, this way one can be sure that they will be having max bills upto $10, again this will be an optional feature..