There is no cheapest way to buy bitcoin however you can buy it from someone who needs urgent cash. The smart way to buy bitcoin is to buy when the price is low. If you see the price was $3700 on 27th nov but now the price is $4,224.38 on 29th nov.
Coinbase is the quickest and cheapest way to buy Bitcoins
You will pay at least 1.49% of your purchase amount as a fee when you buy Bitcoin with Coinbase. It will charge at least 3.99% of your purchase amount if you use a credit or debit card to buy Bitcoin.