Bitcoin transaction fee fluctuates a lot. It depends on how many people want to send their transactions and how many of them want to push them in front of the others.
Basically, your transaction fee also depends on the size of your transaction, but it's mainly the huge amount of traffic on the Bitcoin blockchain that's making it fluctuate so much.
A good site to check what Bitcoin fee to send: https://bitcoinfees.earn.com/
A graph of the Bitcoin Transaction Fee I found: https://bitinfocharts.com/comparison/bitcoin-transactionfees.html
many wallets give an automatic estimate of the fee to be entered for the transaction, but the sites indicated are very useful to set the best fee.
it is important to find the right balance between the lowest fee to be sent and the highest probability of its confirmation!