the 340 ADA cost will be the reward received by POOL (extracted from the total reward) in case they will create blocks
so… this is how rewards are working
no blocks - no rewards
rewards calculation:
1 block = ~700ADA
the rewards which will be shared with the delegators will be = (number of blocks x ~700ADA) - 340ADA pool cost (deducted only once per epoch no matter how many blocks the pool will create) - pool margin
for example if a pool will create for example 3 blocks, cost 340 ADA and pool margin 1%
(3 x 700) - 340 - 17,6 = ~1742 ADA (rewards shared with the delegators)
when you will delegate you will not need to pay 340, you will pay only 2ADA (stake address register and you will receive back when you will deregister the address) + 0,2 ADA transaction fee
Cheers,
PS: I don’t know if I was clear, you can also read this topic