Year	US Dollars adjusted for inflation)
1980	13.88
1981	14.61
1982	14.75
1983	15.34
1984	15.84
1985	15.53
1986	15.95
1987	15.87
1988	16.38
1989	17.14
1990	17.09
1991	17.32
1992	17.35
1993	17.79
1994	17.9
1995	17.92
1996	18.01
1997	18.36
1998	18.52
1999	19.05
2000	19.22
2001	19.57
2002	19.9
2003	20.17
2004	20.07
2005	20.1
2006	20.33