If you want the cheapest, go with Cloudfare. They guarantee to ask for the wholesale price, which is the price the registrar pays to the top level domain owner, so they can't go lower without footing the bill.

What most of registrars do is foot the difference for the first year, so you get a domain for super cheap, then add 50%+ cut on top, so you pay i.e 5$ for the first year for a TLD that has wholesale cost of 10$, while they loose 5$ on that sale, and then you pay 25$ for the second, so they now gain 15$ on holding your domain hostage.

Cloudfare guarantees that they will sell you that domain for 10$, and only raise it when the TLD owner raises the price.

I'm not aware of any other registrar that guarantees wholesale prices, but LMK if anyone knows any.

If you want to get the best deal, buy your first year (or maybe 10 years, if they let you buy 10 for the sale price) with the scammy registrar, i.e get the 5$ sale on Namecheap, and before it expires transfer it to Cloudfare so you don't pay extra for the second year and can continue with the (much lower now) wholesale price.

Each TLD has a different wholesale price, but every registrar pays the same to them for selling the domain to you. The differences you see is exactly in how much are they willing to foot the bill at first (most have a massive sale on first year, then huge markup on renewals), some just add a flat fee and have markup from the start.

Cloudfare just states "you will pay wholesale", and don't do any sheninegans. At least that's how it was last time I checked.