| Year | Price | Year | Price | Year | Price |
|---|---|---|---|---|---|
| 1980 | 3.41 | 1991 | 1.90 | 2002 | 1.86 |
| 1981 | 3.26 | 1992 | 1.82 | 2003 | 1.79 |
| 1982 | 3.15 | 1993 | 1.70 | 2004 | 2.13 |
| 1983 | 2.51 | 1994 | 1.85 | 2005 | 2.60 |
| 1984 | 2.51 | 1995 | 1.68 | 2006 | 2.62 |
| 1985 | 2.46 | 1996 | 1.87 | 2007 | 3.29 |
| 1986 | 1.63 | 1997 | 1.65 | 2008 | 2.10 |
| 1987 | 1.90 | 1998 | 1.50 | 2009 | 2.45 |
| 1988 | 1.77 | 1999 | 1.73 | 2010 | 2.97 |
| 1989 | 1.83 | 2000 | 1.85 | 2011 | 3.80 |
| 1990 | 2.25 | 2001 | 1.40 | 2012 | 3.91 |