£7-800, tops. Apart from the A600 portable, A1200 and A500-040 board it's basically only cheap stuff left.
It's more likely it will rise in value than fall. The value is in the add-ons, unless you have a rare Amiga model. If you're afraid of stuff yellowing, well, there are methods to un-yellow Amigas.
I don't really care if my collection is worth £70 or £7000, I'm not selling it. Maybe some add-on or such to people who need it, but that's it