I didn't read about the salary that was offered to him, but it was probably substantially higher than he would have gotten either at Chelsea or at Inter Milan. But I think he is in a situation in his career where money still isn't everything and he wants to play a few more seasons in the Champions League and win some titles if possible. Not some titles in the Champions League, but in general. I guess he thinks that there were some misunderstandings in recent years that he wants to clean out from his chronology as a player. Inter Milan would be the best destination for him to do that and I still think that both the club and the player will do what they can to sort that out.
If we check statistics in Serie A actually Lukaku did not contribute much to Inter from 25 matches Lukaku scored 10 goals and 6 assists but in the Champions League a little better 8 matches 3 goals overall not too impressive. Lukaku's best career in 2020/2021 when he got several awards after moving to Chelsea his career seemed dim and now he is 30 years old so I thought he only had 3 years to maintain his career, besides the Saudi League and Inter who else wants to approach Lukaku? I have never heard of another club want to bring Lukaku.
I know that it is complicated, but the numbers you mentioned are ok, aren't they? If a player transfers during the winter break I think it is never really easy to perform the best they can immediately. Considering that and the lack of practice he had since he was also sitting on the bench quite a lot before he came back to Inter, scoring 10 goals and giving 6 assists means 16 goal participations in 25 games. He didn't start all of those 25 games either. If he can repeat that for a few years with that average participation per game, I think he would be worth some money.