Year	US Dollars
1972	290
1973	360
1974	450
1990	1240
1991	1290
1992	1500
1993	1520
1994	1510
1995	1750
1996	1860
1997	1880
1998	1660
1999	1640
2000	1580
2001	1610
2002	1480
2003	1600
2004	2020
2005	2870
2006	3050
2007	3190
2008	3050
2009	2980
2010	3070
2011	3570
2012	3930
2013	4010
2014	3670
2015	3280
2016	2960
2017	2960