I don't buy many domains but I just checked the company I usually go to for domains and hosting, they offer domains with free registration and annual renewal rates that come down to ~ 2.5€ per month for TLDs like .app or .dev. Fluctuation on my more common TLD domains either aren't present or not high enough to be noticeable for me in the past. That's what contracts are for around here.

Your post makes buying domains sound like a gamble, is that a regional thing or was I just lucky in the past?