Now that Bob mentioned me saying the "mintage trap", I might as well explain it a little further.
Rarity is the top priority with circulating coins, whether it is a pattern, a bank specimen, a trial strike or a survivor of melting/confiscation/aborted production run. Circulating coins are strictly controlled and regulated by the government. They normally have a humongous mintage. Circulating coins with a small mintage are exceptions and as such they are very special.
With medals, often the mintage is determined by the mint or the private group/person commissioning the medal. The mintage can be manipulated to the advantage of the sponsors/commissioners. It can be artificially very small. If we apply the same rules as those for circulating coins and blindly seek rarity with medals, we may fall into the mintage trap.
I am not saying rarity is not important with medals. The Great Wall silver medals are extremely rare and highly sought after. There are some early brass medals mostly from Shanghai Mint which have a tiny surviving mintage due to loss after their release. (Who cared about brass medals with little melting concept of value?) Their current rarity was not intentional at the time of striking. Before someone decides to fork out a large sum of money for a new medal of a small mintage, beware of the difference between circulating coins and medals. There are tons of such medals of a small mintage out there, and more are on the way. If I see a piedfort version of the Nanjing pandas with an even smaller mintage, I won't be surprised at all. Keeping a perspective on it will save us from falling into the mintage trap.
Just want to give additional insight of “mintage trap” for medal collection.
Collector base:
The valuation of any commercial product is determined by the ratio of supply and demand. For coin collection, the mintage is supply and collector base is demand. Assuming the ratio of number of circulated coin collector and number of PM commemorative coin collector is 20 to 1 and the ratio of PM commemorative coin and medal is also 20 to 1, the valuation of medal with mintage of 100 will be about the same as the PM commemorative coin with a mintage of 2,000 and circulated coin with a mintage of 40,000. (The ratios given herein are for illustration and no scientific base, but the trend, IMO, is correct.)
Brand name:
Brand name takes time and money to establish. For example, currently, 1995 W 1 oz. US silver eagle with a mintage of 30,125 has a valuation of $4,200 (PF69). Its mintage is higher than proof 1 oz. silver pandas (1989-1996) which have a mintage from about 8,000 to 25,000. The current valuation of 1 oz. silver proof pandas (PF69) is about $200 to $600. It is true that the collector base of American Eagle is greater than silver panda, but the brand name definitely also plays a role in this instant.
The quality of Official mint medal vs. mint medal:
Most medals issued prior to 2000 are official mint products which require the approval of the People’s Bank of China or China Gold Coin Inc. After 2000, all third party commissioned medals produced by Shanghai, Shenyang, Shenzhen and Nanjing do not have the approval of the PBOC or CGCI and in practical, be called as “mint medal”, not “official mint medal” as labelled by NGC. It is my understanding that some of the "mint medals" may not made at production lines at mint for coin production. There are differences in quality control process for coin and medal if they are not produced at the same production line. IMO, there is a quality difference between “official mint medal” and “mint medal”.
Attached is an official mint medal issued in 2009 in commemorating the 60th birthday of the founding of the PRC by China Gold Coin Inc. and produced by Shenzhen Mint. This plated copper medal was received as gift when I purchased the coin set. The quality of this medal matches the coins recently produced at Shenzhen Mint.