It's the same for me right now 
Flat rate is 5.5%, Dynamic rate is 5% and I tried entering different amounts of BTC with same result.
In opposite direction when exchanging XMR to BTC fees are much lower, 1% and 0.5%



Small workaround is to use other altcoins first and than swap them to monero.
I think there is a big shortage of monero for a while, but you can search for alternative exchanges on Orangefren and Trocador.