Hate to interrupt a good argument, but I'd suggest looking up transfer fees before you make a point, and then reiterate that point.
Martial cost a total of £43.5m, £36m up front and him hitting one of the three £7.5m clauses (he'll not hit the other two) compared to Lucas's £25m, so not more than twice as much.
When you're buying a "wonderkid" the price reflects that, and it's an entirely different transfer to that of a failed transfer made by a bigger club, hence if we sold Martial now it would be more relevant to the Lucas transfer, when you're shopping at a higher end of the market and you're United things cost more than a Spurs.