He was one of the most expensive GK ever back then too. It wasn't a bargain back then
I think ter Stegen was 8th most expensive or something. Still had Toldo, Buffon, Neuer, de Gea, Handanovic and a couple of others ahead of him.
Lately you have transfers like Ederson for 40m which is
a lot, but then again you have your GK for a decade as Ederson got the talent and mindset to be among the best in the World.
ter Stegen for 12m was a steal compared to that even if he had a year left on his deal. A team like Liverpool could easily pay a WR fee for him now. Arsenal too with Cech fading away and PSG struggling with their GKs. Juventus will have this problem very soon too as Sczescny is nothing more than a stop-gap.
Seem to have become a problem for top teams or aspiring top teams to sign long-term quality GKs which is why Chelsea are offering to pay Courtois a king's ransom to keep him.
Criticized ter Stegen a lot at the start of the season, mainly due to his struggles last season. It's crazy how he's gone from strength to strength this season and what he is displaying consistently. Every flaw or thing he struggled with seem to have disappeared. Perhaps it was just him adjusting after having to cope with irregular minutes for most of his time at Barca. His shot stopping and reflexes improved 10x.