Solid Tube is an interesting case. It was intended as a tube mic in the $1000 price range. Lots of people were unimpressed by it at that price point, and the price point dropped. In 5 or 6 years, it has gone from being an overated $1000 mic to an underated $500 mic. I would check the current exchange rate, but AKG mics aren't marked up in Europe too badly, like some brands. If it's brand new, $600 is a fair price. If it's used, I would expect to pay about $500 for it.
Frankly, I like it. It is pretty dark, kind of like a more colored version of Oktava MK319. It was built for high SPL's, and I think you need to give it a certain level of volume to make its color kick in. I wouldn't use it on a weak singer with a thin voice. It rocks on belters, screamers, and Divas.-Richie