| Year | Price |
|---|---|
| 2015 | $8.43 |
| 2014 | $8.17 |
| 2013 | $8.13 |
| 2012 | $7.96 |
| 2011 | $7.93 |
| 2010 | $7.89 |
| 2009 | $7.50 |
| 2008 | $7.18 |
| 2007 | $6.88 |
| 2006 | $6.55 |
| 2005 | $6.41 |
| 2004 | $6.21 |
| 2003 | $6.03 |
| 2002 | $5.80 |
| 2001 | $5.65 |
| 2000 | $5.39 |
| 1999 | $5.06 |
| 1998 | $4.69 |
| 1997 | $4.59 |
| 1996 | $4.42 |
| 1995 | $4.35 |
| 1994 | $4.08 |
| 1993 | $4.14 |
| 1992 | $4.15 |
| 1991 | $4.21 |
| 1990 | $4.22 |
| 1989* | $3.99 |
| 1988 | $4.11 |
| 1987 | $3.91 |
| 1986 | $3.71 |
| 1985 | $3.55 |
| 1984 | $3.36 |
| 1983 | $3.15 |
| 1982 | $2.94 |
| 1981 | $2.78 |
| 1980 | $2.69 |
| 1979 | $2.47 |
| 1978 | $2.34 |
| 1977 | $2.23 |
| 1976 | $2.13 |
| 1975 | $2.03 |
| 1974 | $1.89 |
| 1971 | $1.65 |
| 1967 | $1.22 |
| 1963 | $0.86 |
| 1958 | $0.68 |
| 1954 | $0.49 |
| 1948 | $0.36 |
*1989 and later figures based on NATO survey; previous years based on CPI-W index