A few years ago one of the bike mags - Classic Bike Guide I think, did an article on the SRX 600 and really rated it. Likening it to a modern Gold Star replacement ??? At the time I was looking for one and as you quite rightly say the perceived values shot up, from £1200 for a clean original bike to around the £2000 mark. Apparently the early twin shock ones are the most desirable and fetch the best prices, and I have seen restored ones advertised for £3000 + The article did mention the 400cc version but said they were underpowered but other than that the same bike, just not worth as much and less desirable. They did warn though, that apart from the engine, parts were difficult to source, especially bodywork and original exhausts were like hens teeth.