Year	U.S. dollars (Units)
1980	232.381
1981	234.940
1982	241.083
1983	247.995
1984	218.999
1985	247.644
1986	253.424
1987	231.932
1988	211.184
1989	213.175
1990	207.113
1991	207.674
1992	187.243
1993	162.572
1994	157.474
1995	167.177
1996	142.588
1997	157.054
1998	141.755
1999	133.412
2000	130.225
2001	127.349
2002	116.392
2003	108.066
2004	122.381
2005	148.710
2006	164.575
2007	170.174
2008	195.790
2009	208.780
2010	231.549
2011	247.097
2012	250.118
2013	255.319
2014	273.133
2015	304.243
2016	281.062
2017	291.859
2018	271.289
2019	261.294
2020	259.906
2021	273.956
2022	310.952
2023	324.904
2024	230.044
2025	239.663
2026	267.079
2027	289.336
2028	309.410
2029	320.783