Right now Porkbun has the cheapest first year at $52.01, and Sav the cheapest renewal at $88.58. Over three years the lowest total at one registrar is $248.71 at Porkbun.
Why does .xxx cost more after the first year?
Many registrars sell the first year below cost and recover it at renewal: at Porkbun, .xxx renews at $98.35 after a $52.01 first year. Compare the renewal and 3-year columns before you buy.