Many of the recent commemorative issued only as 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 BU coin - frosted Proof
1991 South Asian Federation (SAF) Games V - Rs 500 Proof - Rs 100 Proof
1992 Second Executive President R. Premadasa - Rs 1 frosted Proof - Silver Proof
1993 Anubudu Mihindu Jayanthiya - 2300th Anniversary - Rs 500 Proof
1998 Sri Lanka Independence - 50th Anniversary - Rs 5000 Proof - Rs 1000 Proof
1999 Cricket World Cup - 1996 World Cricket Champions - Rs 1000 Proof
1999 Sri Lanka Army - 50th Anniversary - Rs 1 BU coin - frosted Proof
2000 Central Bank of Sri Lanka - 50th Anniversary - Rs 1000 Proof
2000 Sri Lanka Navy - 50th Anniversary - Rs 1 BU coin - frosted Proof
2001 Sri Lanka Air Force - 50th Anniversary - Rs 1 Proof
2006 Buddha Jayanthi 2550 B.E. - Rs 2000 Proof - Rs 1500 Proof
2007 Cricket World Cup - Runners-Up - Rs 1000 BU coin
2008 Employees Provident Fund (EPF) - 50th Anniversary - Rs 1000 BU coin
2009 Sri Lanka Customs Department - 200th Anniversary - Rs 200 Proof
2009 Sri Lanka Army - 60th Anniversary - Rs 1000 Cupro-Nickel BU coin - Silver BU coin
2010 Central Bank of Sri Lanka - 60th Anniversary - Rs 5000 Proof
2011 - Sambuddhatva Jayanti 2600 - Rs 1000 Proof
2011 - People's Bank 50th Anniversary - Rs 1000 Proof
2011 - Ananda College 125th Anniversary - Rs 2000 Proof
2012 - Sri Lanka - Japan 60 Years. - Rs 1000 Proof
2014 - Anagarika Dharmapala 150th Birth Anniversary - Rs 500 Proof
2015 - Pope Francis visit 2015 January 13-15 - Rs 500 Proof.
2015 - Colombo Municipal Council 150th Anniversary - Rs 500 Proof
2017 - Visakha Vidyalaya Centenary Rs 2000 Silver Proof
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 26 years there were 26 NCLT issues. Four in Nickel plated Steel, Four in Cu-Ni, Two in Gold, Three in selective Gold on Silver, and rest in sterling silver. The initial demand was not good for the first two Nickel plated Steel 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 cover cost of minting. 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 was glad there were no NCLT for over 5 years since 2001 March. The NCLT made for the collector market are not real currency. Since then thear has been on average about 1.5 coins each year. 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 for the pair of coins, under 10% of the minted issue was sold in the first 2 years. The price was dropped in 2008 August to Rs9,700 (US$90) for the pair and sold subsequently at price of other Silver crown size coins.
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 lot more than the face value.
The second reason is when the Numismatic Value of a limited or Proof issue exceeds the face value and intrinsic melt value of coin. All of the NCLT listed above are of this type.