I think the Mk1 V6s have taken their initial hit now on price, and residuals will start firming up now that the mk2 is discontinued. I think the Mk2s will drop to close to mk1 prices over the next 2-3 years, but Id be surprised if either mk1 or mk2 drop below the £10k barrier for some years to come, due to their rarety value, & classic status.
Check out the prices of Escort Cosworths - theyre all 8-12 years old now, yet due to their status & rarety value youll struggle to pick one up under £12k, and you can pay nearer to £20k for an unmodified one, and theyve been rock steady at those prices since about 1998, they still dont get any cheaper!
Yet there were about 2500 Mk1 big turbo Escort Cossies made for the UK (cant remember how many small turbo versions, but not as many), as opposed to 400 Mk1 vees and 500 mk2 vees, so I forsee a similar future for V6 values, and there may even be some money to be made as one by one they get scrapped!