I think that the question you are asking is almost too big - akin to "the price of houses" - which house, where, how big etc. etc.
However, If you're talking about GA tourers, once you've got over the intitial depreciation - which is akin to new car economics - age tends to matter less than condition and the value tend to be determined by where the a/c is on it's various maintenance cycles.
There are three main areas of expense; engine, avionics, airframe (bushes, undercarriage, rose-joints etc) paint and interior. Paint may last 10 years if looked after or 5 years if not. Engines generally have a Time Before Overhaul (TBO) of 1,800 or 2,000 hrs - and a further 10 or 20% on condition (depending on the model); with a flying school a/c this may only take 4 years to rack up - with a privately owned aircraft it may take 30. Remember that there is a new EASA requirement that the a/c cannot be used for public transport work if the engine is more than 12 years old. Interiors last for as long as the owner will look after them and avionics deserve a thread all to themselves.
Suffice to say, taking this to absurdum, an aircraft which needs all of the above renewed should be the cheapest of it's kind and an aircraft that has had all of the above recently refurbished/replace/overhauled will represent the top of the market. Therefore, logic would dictate that an aircraft should be worth what the best of it's kind will fetch, less the cost of doing the work.
However, in the world of very aged spam-cans that is largely that of GA, life is not always that simple; given a PA28 140, where the best example can be bought for £30,000, the sums for a knackered example are APPROXIMATELY as follows; £10k for engine overhaul, £5k for paint, £3k for interior and £12k for new nav/com, transponder and DME. This sum should suggest that there are lots of tired old aircraft knocking about for free - which clearly there aren't.
The sum works better with PA28 161's or 181's, where you can buy a tired machine for £25-35k and spend the money and your machine will then resell for £50-60k quite readily.
Another post suggested that the values of the UK GA spam-can fleet were being knocked by the availability of newer, nicer a/c like Cirrus appearing on the s/h market and I suspect that this will is true and will continue to affect values. However, no-one yet knows what it will cost (or if it will be possible) to prolong the life of some of the plastic a/c in the same way as had happened to the old spam-cans. I watch with interest....!
As to negotiability. you have to decide what the individual aircraft is worth independent of what the owner is asking; if you buy the aircraft based simply on the size of discount off the asking price, you are likely to end up buying the aircraft that was most over-priced, rather than the best value for money!
Happy hunting
For in