It was probably one of the least sold models after the TBi.
Not least because by this time, petrol was deemed evil and the tax on these cars was well up to over £400.
The 2.4 Diesel can be well tuned and returns a good economy, and the 2.2 is more typical of what people could afford, especially if they used the car for commuting.
This particular 3.2 is a very classically behaving engine, with an American block, not like the astonishingly efficient 3.0 units you can get in the 3 series or allegedly the new Giulia QV. With this come the consequence of it being a very heavy block, adding to an already heavy car, which can apparently make for some nose heaviness.
Quite frankly, the 3.2 is not a cheap car to run, and petrol owners were being forced out in their thousands by the EU love of diesel. It was never going to be a best seller past 2006 in the UK with the tax changes. BMW had already picked up on this and immediately drove down the consumption on their 3.0 units with the EU nibbling at their tail, and increased the power to easily match that of the 159 3.2 JTS.
Right engine, wrong decade, low sales volumes I'm afraid. You can see the effect on Autotrader and Pistonheads very much. They can typically sell for ~£4000 with good histories. You may find the odd enthusiastic dealer with a Prodrive Brera 3.2 and thinks it's worth £12,000 or so, but compared to other cars in the same class, there is much better.
The 159 TBi for comparison still sells regularly at a lofty £10,000 and higher for a Ti spec (but they are also lower mileage, and younger, but still desirable), as a base price at some used car dealers, and the 2.2 has a wide range because it was the more common petrol engine. The resale value of diesel cars is also quite reasonable for this model, with the uptake being better for people blinkered by diesel, but only do ~5000 miles a year.
I think you are just looking at low numbers of owners really.
*Let the flaming begin*