Always thought he was over rated.
Had a little chuckle when he was 'rated 50M' haha
You shouldn't in general, his progression is first real season had 19 starts, 2 goals 2 assists, second real season had 24 starts, 9 goals 5 assists. That is akin to what Martial got last year and at the same age give or take, 19-ish.
There are exceptionally few players who get 9 goals in their second real season in football, exceptionally few, like 4-5 in the past 15 years. Ronaldo in 03/04 when he was 18-19 got 4 goals 4 assists, with 14 starts 14 subs. at 29 with 25 starts he got 5 goals 4 assists when 19-20. Most players do not score 9 goals in their second season in football.
Aside from that Sterling's play down the left is still very good, where Walcott is all goals and awful awful build up play, the complete absence of ability to beat players while dribbling or pick out passes up and down the pitch Sterling is all that without the end product right now. He's regressed in end product in the past season but he still regularly provides penetration down the wing. He's still only 20 and shooting and end product would generally be expected to improve from say 20-24. The general path most players follow is showing big talent early, but slow growth in end product till at 22-24 depending on number of games played you either have the break out year or not. THe year Bale went from 10 to 21 goals, or Ronaldo from 15 to 30 goals, etc, or you don't have that year.
It's always a gamble, but Sterling's ability in the build up if not the final ball is still very good and what he's achieved at the age/level of experience he has absolutely would dictate a huge fee, boosted by being home grown and the commercial aspect of having an England player.
Is he absolutely worth 50mil, who knows, if he hits 24 and is scoring 15-20 goals a season, yes, if he doesn't then it depends exactly how well he is doing. Martial looks by a very large margin the better player, who also had outstanding(and similar) stats to Sterling at the same age in only his second season. But Sterling could be worth 50mil in 3 years where Martial is worth 100mil.