Year	U.S. dollars (Units)
1980	764.919
1981	764.864
1982	715.135
1983	598.533
1984	477.314
1985	439.344
1986	293.871
1987	353.589
1988	578.180
1989	597.885
1990	542.462
1991	475.448
1992	452.218
1993	431.614
1994	431.567
1995	437.482
1996	404.235
1997	471.141
1998	377.518
1999	353.893
2000	364.026
2001	379.713
2002	399.099
2003	452.287
2004	556.050
2005	720.204
2006	1066.020
2007	1133.672
2008	1393.780
2009	1151.193
2010	1469.239
2011	1644.130
2012	1729.580
2013	1840.665
2014	1724.805
2015	1307.536
2016	1250.324
2017	1495.751
2018	1475.200
2019	1268.125
2020	958.265
2021	1134.713
2022	1454.826
2023	1380.974
2024	1413.421
2025	1493.054
2026	1573.374
2027	1662.791
2028	1756.875
2029	1853.015