In short: no one knows and no one can predict anything about this. Like in some other areas, past behaviors will not give you indications of what will happen in the future.
Any kind of sugar coating on this is just pure personal ramblings.
The facts are that:
- the gTLD registries are under contracts with ICANN, with some variations, for example VeriSign for
.NET still have some historical contracts. The specific case of VeriSign is long and will not fit here. Basically they had the right in the past to change price each year by some percentage and they kind of negotiated to have a presumptive perpetual lease on the TLD by giving up the prices increase (at least for now). See http://domainincite.com/11156-breaking-verisign-loses-right-to-increase-com-prices
- contracts expire and get renewed; sometimes some TLD are put again in a tender process to be awarded to another registry (this is how .ORG was redelegated from VeriSign to PIR). In such cases no guarantees could be made on how prices will stay the same or not.
- newer registries have basically no fees constraints, except those described in article 2.10 of their contract: "Registry Operator shall provide each ICANN accredited registrar [..] advance written notice of any price increase (including as a result of the elimination of any refunds, rebates, discounts, product tying or other programs which had the effect of reducing the price charged to registrars, unless such refunds, rebates, discounts, product tying or other programs are of a limited duration that is clearly and conspicuously disclosed to the registrar when offered) of no less than thirty (30) calendar days.", but also follow up with point b of the contract.
- there are various fees, both fixed and variable that ICANN impose on registries and registrars; these can change too, be waived, etc. See article 6 of this contract for some ideas: https://newgtlds.icann.org/sites/default/files/agreements/agreement-approved-31jul17-en.html
- ccTLDs registries (and your
.ai example falls in this category as does any 2 characters TLD,
AI is the country Anguilla, see https://www.iana.org/domains/root/db/ai.html) are not under contract with ICANN and are even more free to put whatever restrictions and price structures they like and to change at any time. And sometimes quite strong changes, see for example https://medium.com/@Oskar456/stolen-sk-domain-717e070f6735); but you may also get more stability in the sense that a ccTLD is not competing with others ccTLDs, and sometimes with just some gTLDs, where gTLDs are basically competing (brand cases apart) almost all against each other; some or most of them are run by non-profit organizations that may have end-users in their board (so prices are put in control)
In short, registries, especially the newer ones, are between the need to be competitive in a sea of numerous new gTLDs (hence lowering prices typically, as the volume of domain names tend to be the sole indicator of success, which is not without its own peril) and the need to be profitable (hence having sometimes an higher price because of very low number of volume of domain names in the registry), except if this is covered by other services or a company that can run the registry part as a loss.
Registries put aside some names as premium ones, with specific prices (either one-off price at creation and then renewals are at standard prices, or premium prices for both creation and each renewals until the domain name gets deleted, you can see both models in the wild) and they do conduct some regular or one-off promotions. For example,
.EU gives rebates to their registrars depending on their volume increase between two periods (from past observations, such rebates are rarely forwarded to the registrant, but the registrar base prices may be computed based on these rebates from the beginning).
Then, do also remember that you have registrars, competing against each other. On top of registry prices, they put their own margins, that can fluctuate a lot, both depending on where they operate from, the time of the year, the TLD, etc.
So expect a lot of changes and surprises.
Also, not to be disrespectful, but sometimes questions on domain names prices are quite misplaced (your question lacks context about if you are speaking about like personal domain names for no commercial activities, or big portfolios of brand owners, or anything in between): the domain name by itself is almost pointless, you obviously have services on top of it, like webhosting, emails, and so on.
As registrars have shown, nowadays the domain name price just by itself, and excluding premium ones of course, is just a tiny amount of the whole package of services you need to pay to be online. So when cost lowering or control is needed, domain names may not be the first and best spot to try winning some money. But of course, at least on the registrar level, you have competition so you are free to change if you find a better one (I would advise however not taking just the price point as a unique comparison tool between them; also not using the same registrar for all domains can be interesting sometimes).