In 5 years, the bog average PL footballer will be valued around 50 million. Lukaku will be about about 250 million.
In today's money, (discounting at football inflation rate of say 20%)that is about 100 million.
But the TV money at the end of the next deal will only increase the inflation rate. I suggest each PL team (averaged over the 20) should receive in the vicinity of 225 million pounds TV money post 2019.
Thus , a price tag of 100 million quid on Lukaku is probably minimum 'fair' economic value.
. This might sound high, but soon, such fees will be commonplace
(And I am being conservative in my monetary expansion predictions above. China's central Bank is printing money like confetti...15 trillion USD equivalent since 2009. They will make footballers the richest sportspeople on the planet. When money is free, it doesn't cost anything to buy things/real assets with it.)
PPS whilst all the inflation is rampant, bank deposits will still only yield 2% per annum. Funny old world.