In Ronaldo's case also, he is not getting the entire amount upfront. $250 million will be paid over several installments, during his stay with Al Nassr. And then there is another payment for $250 million (so the total goes to $500 million), which is linked to Ronaldo being a part of the Saudi team, which will bid to host the 2030 World Cup. Usually the bid team includes only citizens of that particular country and that is the reason why there is speculation on Ronaldo accepting citizenship from Saudi Arabia. But we need to remember here that dual citizenship is not recognized in Saudi Arabia and therefore if he gets the Saudi passport, then Ronaldo need to forfeit the Portuguese nationality.
It is not as low as Beckhams neither though, this one is for 75 million a year now matter what, and that is a ton of money, plus the extra that you are talking about. Basically what Beckham had was 30 million or so plus the incentives, and he got the incentives, and in this case we are talking about 75 million plus incentives, so if Ronaldo does get it like Beckham did, it will be 100+ million a year, not similar.
Obviously there are some calculations to be made regarding inflation, and maybe they are quite close to each other but I honestly believe that we are talking about a situation where it is much bigger payment for Ronaldo, doesn't look even similar to me.