Difficult question, because it's hard to determine who is better or who is worse. I feel like that's a thing with strikers, because one season they're great, others they're nowhere... in a worse sense than any other kind of players. I mean, look at Falcao, Benteke, Torres, Gomez, Jelavic, Mandzukic, Michu, Van Persie, Costa, Rooney (not a striker anymore now) and so on...
The fact that Aguero, Luis Suarez and Ibrahimovic are so consistent every season, that to me is what makes them stand out above the rest. And to be fair, Lukaku suffers a lot of off-days, but overall throughout a whole season, and season after season, he does perform more or less consistently... Maybe THAT is the reason for his super high market price (along with his age). Investing in Lukaku has a high probability of being safe.
So what price would I pay to have that same degree of high-goalscoring consistency... I'd say... Whatever Ibrahimovic' price was. He's old, but he's still better than Lukaku. It's all hypothetical, he's off to ManU, i know.