tommysvr
Vedran Corluka
Trossard is an experienced international and proven PL player. Porro is a promising young player, playing in a 2nd rate league. Also Trossard is an attacker, Porro's a defender, and attackers are usually more expensive. It's a massive difference between the two.
Length of contract was probably the biggest factor in Trossard’s fee. If he had 2+ years on it, he’d probably have been £40-50m easily, for the reasons you mention.