A better definition of "got off easy" would be "without much hardship or cost". That can mean that you didn't get punished very severely, but in this context it means that whomever was speaking got a good deal on the Quidditch World Cup tickets they bought.
The phrase "got off easy" has the connotation that the usual consequence or cost is much more unpleasant or costly than the actual consequence/price in this situation. The tickets could have cost a lot more, but he "got off easy" with a lower price.
Some examples:
She might have gone to jail for a long time, but got off easy with just community service.
I could have ended up in the hospital, but because I was driving slowly I got off easy with just bumps and bruises.
I didn’t know it at the time, but apparently, I got off easy. When my best friend from college got married last summer, I had to spend only $1,000 total for her bachelorette party in Las Vegas, my plane ticket from New York to Los Angeles, hotel room, bridesmaid’s dress, shoes, hair styling and wedding gift.
(Source: HuffPost)
