The g21 short frame rtf2 isn't a very common model. Most were issued to police departments with fewer released for civilian sale. One of my local shops has a handful of new ones for sale for 599 with standard sights. Police trade ins would be worth less to most people. Some of them had a true ambidextrous mag release that can add value.
I saw one at a dealers table at the gun show last weekend. It was $529.00 NIB.
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“Neither cold, nor darkness will deter good people from hastening to the dreadful place to quench the flame. They do it not for the sake of reward or fame; but they have a reward in themselves, and they love one another.”
Benjamin Franklin
At first they were an over-run for a department and only a few thousand were released. At that time, they were going for up to $800. Recently, Glock started producing them for the civilian market and they can be found easily and plentifully. Hence, they are worth about the same as any other G21 now.