Game prices are part rarity, part demand. Pokémon games aren't rare, but they're always in demand so they always command decent prices. Plus even if millions of copies were sold the majority might not be English versions which are often most popular. Chrono Trigger for example sold millions in Japan and can easily be picked up for less than $20 in Japanese. US copies sold more like 500,000 and collectors outside the US are interested as well since English is much more of a common 1st or 2nd language than Japanese.