It's a huge commission, but it probably came from the fact that you paid directly with a credit or debit card..?
Commissions are normally lower, here you have information from Coinbase:
Coinbase charges a Spread of about one-half of one percent (0.50%) for Digital Currency Purchases and Digital Currency Sales. However, the actual Spread may be higher or lower due to market fluctuations in the price of Digital Currencies on Coinbase Pro between the time we quote a price and the time when the order executes.
We also charge a Coinbase Fee (in addition to the Spread), which is the greater of (a) a flat fee or (b) a variable percentage fee determined by region, product feature and payment type. The flat fees are set forth below:
If the total transaction amount is less than or equal to $10, the fee is $0.99.
If the total transaction amount is more than $10 but less than or equal to $25, the fee is $1.49.
If the total transaction amount is more than $25 but less than or equal to $50, the fee is $1.99.
If the total transaction amount is more than $50 but less than or equal to $200, the fee is $2.99.
For example, if you are in the United States and wish to purchase $100 of bitcoin and pay with a U.S. bank account or your USD Wallet, the flat fee would be calculated as $2.99. As noted below in the variable fee section, the variable percentage fee would be 1.49% of the total transaction, or $1.49. Since the flat fee is greater than 1.49% of the total transaction, your fee would $2.99. If you wanted to purchase bitcoin with a credit or debit card, we would charge a fee of 3.99% since the variable percentage fee is higher than the flat fee.
Variable percentage fee structure by location and payment method are shown in the last section below. These fees do not apply to Digital Currency Conversions. As a reminder, the Coinbase Fee will always be the greater of the minimum flat fees described above or the variable fees described below.