It's Queen's most famous song and was written by the band's lead singer Freddie Mercury. It doesn't have a chorus and instead has three parts: a ballad ending with a guitar solo, an opera passage, and it ends with hard rock.
The reason for the single being so expensive probably lies with the fact that it was a truly unprecedented song. Many people consider it ahead of its time because of how complex some of the techniques they used to record it.
The band rehearsed for 3 weeks before recording it, and the whole song took 3 weeks to record after that! The song wasn't recorded all at once, but in different sections. Mind you, some of these sections weren't just at different times of the song, but also different layers of the song.
The vocals were also extremely complex. Some of the opera chorus vocals were recorded continually for 10 to 12 hours a day, and they pushed the limits of what the current recording tapes allowed them to do. To give you an example of the kind of stuff they did, they had 120 vocal tracks that they had to fit in 24-track tapes.
In the end, though the result was one of the best songs of all time, and a song that has stayed in public consciousness until this day.