Seeing that you always refer to Coinbase and now provide a step-by-step guide buying with it, you’re probably payed by or working for Coinbase….
Coinbase and other simple exchanges like Changelly charge ridiculously high conversion and transfer fees.
The cheapest way is to register at a large exchange like Kraken, Bittrex or Poloniex and send fiat to your account to buy BTC from or even ETH/XRP or others. Kraken offers fiat to crypto exchange on most cryptocurrencies.
If you don’t want to leave your cryptos on the exchange you can then send them to a wallet of your choosing.