单位：美元/千立方英尺
| 年份 | 一月 | 二月 | 三月 | 四月 | 五月 | 六月 | 七月 | 八月 | 九月 | 十月 | 十一月 | 十二月 |
| 1997 | 4.25 | 4.29 | 4.22 | 4.06 | 3.98 | 4.22 | ||||||
| 2001 | 4.69 | 4.74 | 4.71 | 4.26 | 4.23 | 4.29 | 4.28 | 4.3 | 4.4 | 4.41 | 4.3 | 4.3 |
| 2002 | 4.27 | 4.04 | 3.74 | 3.68 | 3.79 | 3.85 | 4.09 | 4.26 | 4.3 | 4.28 | 4.3 | 4.34 |
| 2003 | 4.43 | 4.44 | 4.3 | 4.44 | 4.62 | 4.76 | 4.67 | 4.43 | 4.4 | 4.4 | 4.45 | 4.51 |
| 2004 | 4.42 | 4.53 | 4.6 | 4.79 | 4.89 | 4.83 | 4.98 | 5.05 | 5.24 | 5.24 | 5.32 | 5.4 |
| 2005 | 5.27 | 5.4 | 5.26 | 5.19 | 5.38 | 5.49 | 5.89 | 6.08 | 5.97 | 6.24 | 6.4 | 6.68 |
| 2006 | 5.9 | 5.81 | 5.69 | 5.76 | 5.95 | 5.94 | 5.95 | 6.17 | 6.22 | 6.28 | 6.45 | 6.44 |
| 2007 | 6.08 | 5.91 | 5.85 | 5.86 | 5.65 | 5.73 | 5.87 | 6.06 | 6.14 | 9.34 | 6.33 | 6.28 |
| 2008 | 6.6 | 6.9 | 7.09 | 7.09 | 7.21 | 7.38 | 7.49 | 7.97 | 8.56 | 8.72 | 8.87 | 8.27 |
| 2009 | 7.54 | 6 | 6.42 | 7.33 | 7.2 | 7.65 | 8 | 8.36 | 10.2 | 10.97 | 10.69 | 9.18 |
| 2010 | 11.68 | 7.97 | 12.36 | 12.21 | 11.94 | 8.27 | 12.83 | 13.19 | 9.85 | 9.04 | 8.18 | 8.5 |
| 2011 | 8.21 | 9.68 | 10.2 | 8.27 | 8.65 | 11.44 | 12.72 |
