I second the choice of the Voigtlander Nokton 50mm f1.5, especially if you are looking for something sharper and more contrasty. You should be able to find one less than 250€ in LTM mount. The only noticeable difference is the LTM version focuses down to 0.9m, compared to 0.7m of the M mount version. But hey, it's getting closer than the collapsible Summicron already.
The Cosina Zeiss 50mm Planar is also a good choice, as I've been seeing them sold less than 500€, which is considerably less than the price of the Sonnar 50mm. Having used both, the Planar is to me a better lens. (I don't quite understand why people are paying so much for a lens affected by the focus shift issue, and for some reason, more so than other Sonnar type lenses such as the Canon 50mm f1.5 LTM.)
You should consider the Summicron 50mm version III, too. Although more people are realising it's not a bad lens, it still costs less than later versions. I recently bought one for 600€, in good condition with hood. Another auction ended at 670€ with box and hood, and the lens was in as new condition. In my opinion, this lens handles better and feels more solid than the Planar, and it is even less prone to flares than the version 4 Summicron. However, when it comes to sharpness, I doubt there are many lenses on the market sharper than the Planar.
As to the contrast, I find the Summicron 50mm version III is slightly less contrasty than the Planar and the Nokton. But the difference becomes irrelevant if you compare them to old lenses like the collapsible Summicron.
I like the collapsible Summicron, but I have to say even with perfect glasses and without any haze, it provides less contrast and is more prone to flares than the lenses mentionned above. It is a fact that neither DAG nor anybody else can change. It's just if your copy is affected with cleaning marks and haze, the difference would be much more noticeable. I find the collapsible Summicron very sharp between f4 and f11, and perfectly acceptable at f2. But the other lenses will be sharper from f2 to f4.
I hope it helps.