I use mythic beasts. They are not the very cheapest, but they offer predictable pricing and just charge a fixed increase compared to the price they pay their supplier. You can trust that they won’t mess around with the renewal price or arbitrary extra fees.

For my .org domains I pay ~£15 per year, but if you don’t care about the tld, you can get some for ~£6 per year (the costs on the website exclude VAT, but if you buy multiple years at a time, the amortised cost including VAT ~= the price excluding VAT).