Messi costs 175 million Euros for 3 years at the most. Lukaku costs probably around 200 million Euros or even higher. 115 mio. Euros is just the fee, I expect couple of millions agent fees (let's say 5 million Euros) and he will probably earn around 24 million without tax (so 12 Mio. net salary approximately, which Fabrizio Romano said will be his salary after tax) and this for 5 years (120 million for 5 years in salary cost). So Lukaku costs 240 million Euros for 5 years in total, while Messi costs 175 million for 3 years.
Many people will buy a Messi shirt, while I expect that not that many will buy a Lukaku shirt. Messi also means new sponsors. I highly doubt we will attract new sponsors because of Lukaku. PSG made a good deal, if their finances are solid.