why they charge 50$ on 125$ worth of bitcoin
and 50$ charge on 20,000$ worth of bitcoin
is because rich look after the rich, always have always will
It may sound ridiculous but it's the traders who offers the fee i.e. the fee is decided by the person who makes the transaction not the miner.
Miner just selects who's job to be taken and they generally select the one with higher reward.