Sign of the times.
In the early 90's my father was working in Sweden and bought a sierra xr4i to go with. At the time you could buy a sapphire cosworth for little more (about £4K at the time for a c3-4yr old one). He chose not to purely because the one he wanted could not be prepped in time for him to take away.
These are now fetching £50k plus in some cases.
Similarly, a friend's father scrapped a number of jaguar e-types in the early 80's when no one wanted them. All cars have their time, take a massive slump and then some rise again. I have no idea if omegas will rise again, but they are certainly having their slump now!
They sure are.
I've seen many 3.2 Elites for example for around £200 - £500 price range and some look pretty good from the pictures (this year).
When i bought one of my 3.2 Elites around May 2013 it was £700. So they have definitely been going down in value. Even ones in pretty good overall condition examples are now around £600-£1,000 where as a few years ago you'd be looking around £1,500 - £2,000.
How much cheaper will they get I can't say? ...I'd like to think the nice looked after examples will gradually increase in value soon. We will just have to wait and see...
Then again take the Senator for example. They are far and few between especially clean looked after examples. I did find this one for just under £7,000 and it does look good. So maybe there is hope for the Omega for example in similar condition in the future.
http://www.ebay.co.uk/itm/Vauxhall-Senator-3-0-CD-24v-1993-52k-Only-Stunning-Condition-/132286581359?hash=item1ecce50a6f:g:HuwAAOSwr7tZh0sV
.... Oh and the white Astra J in question had a baby on board sign swinging away on the back screen 
