How to ensure I always buy an informative domain in the lowest price possible?
You can't, specifically if you aim to guarantee future lowest prices. End of story.
You seem to have multiple questions recently all around TLD pricing, always with some strange views or references to one specific TLD that does not have a place in most questions.
The domain registration business does not work the way you seem to think it does; In short, prices are free, and can change (mostly) at any time. This mostly reflects the economical worldview most countries currently have which is, as you know, supply and demand - putting aside the different moral views about it.
There are restrictions (gTLDs are under contract with ICANN, ccTLDs operators are under some kind of contracts with the respective governments, all of these contracts may have sections about prices or price changes), but to summarize for end users the fact is that no price is guaranteed, neither increase nor decrease nor time frames.
If you want a low price at any given time, you shop around then and do your best. Whatever TLD you choose you have no way to be guaranteed its future renewal price, the registry can change it at any time after your registration.
.ORG sell from one non-profit to a for profit company, and the related ICANN contract change just before that allows basically the registry to increase prices as it wish (and some very strange interviews from people in charge show that they claim that 10% increase per year is "customary" and "not a problem") is an example of the kind of unsettling situation indeed we can be at: everyone having capitalized on its
.ORG domain name is now in jeopardy as it can have its prices increase (for no real technical reason), without a realistic way now to switch to another domain. See https://www.eff.org/press/releases/eff-icann-stop-org-domain-registry-sale-private-equity-firm for one example of people pushing back against this.
Typically, gTLDs registries increase their domain name prices regularly (like yearly or less), because they can and market that for better services and security. ccTLDs may decrease prices because some are under contract that forces them to be run in a non-profit fashion (ex:
.FR) but even in this case you can have creative accounting or starting a foundation as many ccTLDs did, that basically means the price can not be reduced, and has to be increased each year even if the volume of domain names increases too.
On the contrary, when there are huge promotions (like Afilias did back in the days for
.INFO) where basically a domain name can be brought for close to 0, this is not necessarily good (for this specific case, a lot of spammers and nefarious users jumped on the occasion to have many throw away domains, which had the consequences that
.INFO got a lot of bad press at least, and was even completely blocked by some sysadmins at worst), and certainly not something that will last (that promotion was for one year registration, and one year later you had to pay the renewal at normal prices and the TLD dropped its volume significantly).
Also, as discussed on other questions already, you have specific cases like EAP (higher prices at the start of a TLD) or premiums (higher prices either just at registration and not at renewals later, or higher prices just for any future operations), and for all of this again you just have to shop around and be knowledgeable.
Why would things be different for domain names than anything else? If you buy a web hosting package don't you have the exact same question (and the exact same answer even in larger proportions)?
If you want to believe it, some companies acting as registrars, claimed they will deliver domain name registrations "at cost", without margins. One may still have to see this claim be delivered as for now it is not, and one has then to clearly think about how the company runs if it does not make money out of domain names.
purchasing gTLDs might often be somewhat different than purchasing ccTLDs.
There are no real user-visible differences besides the underlying contracts as explained above.
You can read ICANN master contract with registries (the 2012 batch, the legacy ones like
org, etc. have their own specific contract) at https://newgtlds.icann.org/sites/default/files/agreements/agreement-approved-31jul17-en.html
About pricing, look specifically at section §2.10.
As for ccTLDs, you would need to get access to the contract between the relevant government and the TLD operator, which is not necessarily public. Relevant sections of it may be incorporated in the registry-registrar agreement (which may or may not be public either) and again parts of it may be included in the registrar-end client agreement, which you agree to when buying the domain, so you should read it before buying any name.