I just tried to buy tickets from Portland, OR to Madrid, Spain and back as a gift to my 17 year old nephew.
The price online was $1107 but the Delta website wouldn't allow me to complete the purchase, and said that I had to complete the purchase by phone because he was an unaccompanied minor (more about that later).
When I called to buy the tickets, the agent quoted me a price of $1900+. When I said that was the wrong price, she told me that WHEN YOU BUY BY PHONE THE SYSTEM MAKES THEM TRY TO CHARGE YOU A HIGHER PRICE (!) and then sold them to me at the $1106.
I guess the idea is to rip off the customers who are too naive to check prices online.
Moreover, it turned out that 17-year olds are NOT unaccompanied minors. The idea of making me buy over the phone was to give them the opportunity to try to sell me an optional so-called "Unaccompanied Minor Service" for $150 extra each way.