Transmission lines increase the end customer costs (a lot). They are necessary but you want as little of them as you can get away with. Obviously not having enough can also increase costs as you have to use very expensive forms of production to compensate.

Here in Finland the transmission costs before this current price hike were usually 50%+ (for me with my current fixed price contract they are around 60%) of the total electricity bill. We got a really good network but it does come with a cost.