Many of the recent commemorative issues are Non Circulating Legal Tender (NCLT) and since 1991 have been sold at a premium by the Central Bank. See also webpage of commemorative coins that circulated, and some comments.
1990 Central Bank of Sri Lanka - 40th Anniversary - Rs 500 coin
1991 South Asian Federation (SAF) Games V - Rs 500 coin - Rs 100 coin
1993 Anubudu Mihindu Jayanthiya - 2300th Anniversary - Rs 500 coin
1998 Sri Lanka Independence - 50th Anniversary - Rs 5000 coin - Rs 1000 coin
1999 Cricket World Cup - 1996 World Cricket Champions - Rs 1000 coin
1999 Sri Lanka Army - 50th Anniversary - Rs 1 coin
2000 Central Bank of Sri Lanka - 50th Anniversary - Rs 1000 coin
2000 Sri Lanka Navy - 50th Anniversary - Rs 1 coin
2001 Sri Lanka Air Force - 50th Anniversary - Rs 1 coin
2006 Buddha Jayanthi 2550 B.E. - Rs 2000 coin - Rs 1500 coin
Lanka issued it's first NCLT only in 1990 for the 40th Anniversery of CBSL. Since it was issued at face value, it could have circulated. In 1991 CBSL obtained authority from the Monetary Board to sell commemorative NCLT coins, at a price higher than face value to cover the cost of production. Such coins were first Minted in 1991 for the SAF V games as Proof Coins; a Rs500/- in gold and Rs100/- in Silver. Over the next 11 years there were 11 NCLT issues. Three in Cu-Ni, Two in Gold, and rest in sterling silver. The demand for the first two NCLT in Cu-Ni Rs1 coins issued with a mintage of 135,000 and 22,000 for Army and Navy and sold at few hundred times the face value to cost of minting did not sell well. Therefore only 2,000 Proof was minted for AirForce of which 800 were sold to public. It sold out, and now a very difficult and costly coin to find.
I am glad there were no NCLT for over 5 years since 2001 March. The NCLT made for the collector market are not real currency. In 2006 May CBSL issued two crown size silver NCLT for the 2550 Buddja Jayanthi. It is rumoured to have been sold at about 30% above cost of production. However because of the Rs12,000 (US$120) price under 10% of the minted issue sold in the first 2 Weeks.
A coin would be NCLT for two reasons. A purely Financial reason would be if the intrinsic worth (Bullion Value) of a coin is exceeds its face value. Such a coin will stop circulating and risk getting melted unless the Numismatic value exceeds the Bullion Value. Because of inflation many low denomination coins that once circulated have now stopped since the value of Aluminum in the coin exceeds the face value. The Central Bank of Sri Lanka, no longer issues these coins, and will do so only with the special request and in limited amounts since minting more coins costs more than the face value.
The second reason is when the Numismatic Value of a limited or Proof issue exceeds the face value of coin. All of the NCLT listed above are of this type.