According to Wiki: '55,573 killed out of a total of 125,000 aircrew (a 44.4% death rate)'.
RAF Bomber Command - Wikipedia, the free encyclopedia
Only the U-boat crews had a worse loss rate (28,000 of 40,000 - 70%).
My dad flew with Bomber Command and did 29 ops in 1942/43. I asked a squadron historian who had the records if there was anything noteworthy about my father's operations. 'No', he said, 'except he survived'.