Huge demand meant that the prices rose to ridiculous amounts in the second-hand market. Now, two years later, the NES Classic is back and the supply has been far better.
The demand is still there which is proven by the fact that according to market research firm NPD it has outsold all other consoles, including the current generation favorites Xbox One, PlayStation 4 and Nintendo Switch.
This even though NES Classic was only sold for a few days during the analyzed period (June 2018). Of course one has to remember that NES Classic costs $59 while the others go for several hundreds.
So as the tweet by NPD analyst Mat Piscatella below shows, NES Classic outsold PS4 by unit numbers but not in dollar sales. If you want to know more about the numbers of both software and hardware sales head to Twitter and read the thread.
US NPD HW - The NES Classic was June 2018's highest unit-selling hardware platform, while the PlayStation 4 led the market in dollar sales. This is the first time a Nintendo Entertainment System console has led in monthly unit sales since NPD tracking began in 1995.-- Mat Piscatella (@MatPiscatella) August 1, 2018