Oh, interesting topic.
I'm no car dealer, but I do web development work for some traders, which in turn gives me access to some industry wide data.
From what I've seen, recommended trade-in values haven't changed that much, but the resale values have gone up by 10-25% for what the data suppliers term 'wide-body' versions. Annoyingly (for me and my '98 widebody 2.8) they include the facelift 4 pots in that group, where as I like to call the facelifts 'mid-bodys'.
Now, whilst this sounds great for existing Z owners, I think it's actually more of a result of legislation changes a few years ago that define that cars sold by traders have to be roadworthy, with a 12 month warranty etc..., and since these cars are getting on a bit, that overhead gets bigger each year.
The result of that on the private market is that prices end up all over the place.
Some people may have bought a car for some value, then want to sell it a few years later, hoping to break even.
Others may ask for a trade in value, and try to sell for a bit more privately, or need to sell fast to release money or fund their next toy quickly.
Then there's people who look at the market and pitch their car amongst the others out there regardless of whether it's being sold with dealer benefits or not.
Then, I also think it's fair to consider the Z3 as an enthusiast car, and as such there is a minority group of potential buyers that will pay premiums for certain must-have spec's, colour, trim, originality, OEM+ spec additions, history and of course mileage and overall condition are always a factor.
We can also see the average advert duration, and with any droptop, seasonal fluctuations do occur. Spring, Summer months prices up and faster sales, Autumn Winter, prices down and slower sales, especially with higher priced stuff which goes stagnant over the winter.
In summary, I can't figure out if actual prices (private sales or net trade profits) are going up or not, but they aren't going down!