There is no real "book" price on these. Not like a 3 year old car or similar.
Not sure where you got those figures from?
I'd expect £3,500 upwards to be realistic for a reasonable one.
Good ones are not falling out the trees, and a cheap V6 GTV banger at 1200/1400 quid will drain your wallet quicker than you can say "scrap yard".
If you want to spend that sort of money, look for a reasonable 2.0TS GTV.
Frankly, I would say the V6 has been to the bottom and now rising quite nicely, as they are in my opinion the most desirable GTV.