For me it was probably the 1.4, 50mm Summilux currently on my M2. I can’t remember what I paid for it, as it was a long time ago and I also traded in a Leica II with a Taylor Hobson 2” lens to get it (that bit was probably a big mistake) . My girlfriend at that time was with me when I bought it and she left me soon afterwards. I don’t think she could understand why anyone in their right mind would spend that much money on something as trivial as a camera lens. However, I have never regretted that purchase.