Aubameyang, if you believe the reports, are probably going to be more expensive for United, in terms of total transfer fee and wages, especially the latter. Even PSG supposedly bulked at his demands. Auba has great scoring records, but I think his game relies heavily on his pace, which I am not sure would be good short term investments given that pace would be the first thing to go.
Immobile? I won't even go there. He failed at Dortmund. I don't want United to buy "Dortmund reject". Plus, "Madrid reject" sounds fancier.
Icardi, he is good player with good records. Interesting prospect, but not sure about him being cheaper, considering when they evaluate "average Perisic" at 50M. At the very least, he would command similar fee as he is not a "reject" and very much an important element to Inter's first 11, and captain to boot.