Companies like Nike have huge research, design, marketing and sponsorship budgets, so like it or not, you are also paying for that. Obviously the fakers don't have that, plus are probably a bit more questionable over workers rights and treatment (debatable, agreed). So 20 quid is unrealistic...
I'm assuming these fake companies can source the exact same materials as the real deal? If the fakes are over 30 quid, I actually think this makes the real ones OK value. Would expect fakes to be more like 20 quid.