...I assure you that the extra cash you pay for Contax lenses doesn´t turn into better pictures. ...
It depends on your definition of a 'better picture'. I also have had many Contax (28/2.8AE and MM, 45/2.8, 50/1.4AE and MM, 50/1.7, 85/1.4, 135/2.0 and 180/2.8) and Yashica (21/3.5, 24/2.8, 50/1.4, 50/2, 55/2.8 macro, 200/4 Yashica ML) lenses and have consistently found the Contax lenses to be sharper and with fewer aberrations than the Yashica equivalents, but some times the difference is very small. The T* multi coating on Contax lenses is superior to the coatings on Yashica lenses and this helps to give better contrast overall and fewer issues with flare.
So whilst Contax lenses certainly won't make you a better
photographer I would argue that they would give you technically better pictures, with few aberrations. As for the cost and whether it's worth paying more, well that's totally subjective.