Inter paid quite a large sum for romelu, and I'm quite surprised at that, lukaku hasn't been on top form since moving to united, but the transfer market this days is filled with high profile amounts for players not worth that particular price per se.
It means that Inter value Lukaku highly and United don't want to lose Lukaku for cheap. Both of them agreed on the deal so that's that. If Lukaku can't hit his top form again, I don't think it's possible for any team to sell Lukaku for more than half of his price right now.
Would be interesting to see how he performs in Inter. Hopefully, he can get his best form again and compete with Ronaldo so that the Italian league would be interesting to watch.