I presume that turbo market may not see a reduction in prices, if not a marginal drop due rarity and niche demand for English localized pc engine games
but pc engine prices have significantly dropped since everdrive flash card release and most games are more affordable than lets say 10 years ago or even 5 years, only really rare games keeps there prices ( reflects hardcore collectors market), you will always get extortion prices on ebay, but auctions will reflect the market
emulation on current consoles has also contributed to the drop as well