Before I bought my Troll. I started keeping a spreadsheet (a bit nerdy, I know) that contained details of every advert I found - model, year, price and the URL of the advert (so I could find it again!). Actually, I had 2 versions - 1 for adverts found in the UK, one for those in Holland. When I had sufficient data, I was able to plot a graph of prices for each model by year which gave me an excellent guide as to prices and also pointed out 'outliers' in the general price pattern, hinting at vans that could be considered 'bargains. As a result, I snapped up my perfect van in my price range...
hth...