It is also just an apparently unavoidable fact that all the PF interest in uncovering and posting bags and thus promoting their sale, along with the passage of time, of course, is driving up the prices. I guess prices become whatever the market will bear. Great if you're selling, bad if you're buying. I admit I virtually never sell, so I have more of a buyer's POV.
If I remember correctly, around 2004-05, if you missed the boat when bags came out, it was routine to pay a re-seller way over the retail price for a brand new one, and over retail for a used but mint/excellent bag. How high the price was depended on condition and the perceived rarity of the color. Rarity probably has a lot to do with popularity and with buyers KEEPING certain colors, as well as actual rarity in production. If I remember correctly, $1200 was pretty normal for a new twiggy; $1400-1500 for a city. Seller's market.
The resale Balenciaga market was then awful for quite some time, as the new leather, more mass-produced bags started coming out and people could more readily obtain brand new bags in new colors at RETAIL prices. It has taken a while for a cycle to happen, in which more and more new devotees became very educated about the "old" leather and colors, what is scarce, etc. For a while, it was only when a long-time, "early days" Balenciaga seller (with an unquestioned reputation for selling authentic, fantastic condition bags) put a near perfect older bag on ebay that these kinds of prices were paid. Now, very many 2005 bags, for example, seem to sell at the high prices formerly reserved for the "very rare." Maybe we have Balenciaga to thank for that, as all the older leather starts seeming better and better. ?? And ourselves as buyers. It's hard to get used to, especially if you are looking for a bargain.
I personally love and adore 2005 turquoise. It was a plentiful color (in comparison with other colors of the season) when it was sold in 2005. I had no problem buying a city at retail. The same is true of another lovely color, sky blue. I remember not so long ago being totally unable, to a group of hard-core Balenciaga fans, to sell a BNWT (including luisaviaroma security tag) sky blue twiggy with to die for leather at slightly above the 2005 retail price. Times change. I paid this kind of price about a year and a half ago for a brand new and unused turquoise 2004 twiggy. I have paid a little more for a BNWT rose city or brand new 2004 turquoise voyage. Now it is getting to be the norm. The market probably won't "fall" again until the prices just go so over the top that only the rare person will pay them!