I'm saying that we know from earlier text that you get n attacks in a round.
If you wield a second weapon in your off-hand, we can make that n+1.
And that if the condition in the first sentence is fulfilled - that is, if we are fighting 'in this way' - a penalty applies.
What is that condition? It's either 'you wield a second weapon in your off-hand', or 'you make an extra attack'.
By my reading, making an extra attack does not describe a 'way' of fighting. It's a benefit of a way of fighting. The way of fighting is using two weapons. So to me, the answer to 'What condition does "in this way" describe?" is 'You wield a second weapon in your off-hand'. Fighting in this way has a benefit - you can, if you so wish, make an extra attack.
It also has a drawback - you take a penalty on your attack rolls.
I don't think that (c) is necessarily 'in return for' (b).
The pattern is, rather: If you (a)______, you can (but need not) get (b)______. If the condition in the first sentence is met, the penalty (c)______ applies.
(c) is not definitively linked to (b); to me, by the PHB wording, it's more intuitively linked to (a), though that is not explicitly defined either.
-Hyp.