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| | 2004 | 2005 | 2006 | 2007 | 2008 | 2009 | 2010 | 2011 | 2012 | 2013 |
| Jan | $6.15 | $6.21 | $11.43 | $5.84 | $7.17 | $6.14 | $5.81 | $4.22 | $3.08 | $3.35 |
| Feb | $5.78 | $6.29 | $8.40 | $6.92 | $8.00 | $4.48 | $5.27 | $4.32 | $2.68 | $3.23 |
| Mar | $5.15 | $6.30 | $7.11 | $7.55 | $8.93 | $4.06 | $4.82 | $3.79 | $2.45 | $3.43 |
| Apr | $5.37 | $7.32 | $7.23 | $7.56 | $9.58 | $3.63 | $3.84 | $4.24 | $2.19 | $3.98 |
| May | $5.94 | $6.75 | $7.20 | $7.51 | $11.28 | $3.32 | $4.27 | $4.38 | $2.04 | $4.15 |
| Jun | $6.68 | $6.12 | $5.93 | $7.59 | $11.92 | $3.54 | $4.16 | $4.33 | $2.43 | |
| Jul | $6.14 | $6.98 | $5.89 | $6.93 | $13.11 | $3.95 | $4.72 | $4.36 | $2.77 | |
| Aug | $6.05 | $7.65 | $7.04 | $6.11 | $9.22 | $3.38 | $4.77 | $4.37 | $3.01 | |
| Sep | $5.08 | $10.85 | $6.82 | $5.43 | $8.39 | $2.84 | $3.65 | $3.86 | $2.63 | |
| Oct | $5.72 | $13.91 | $4.20 | $6.42 | $7.47 | $3.73 | $3.84 | $3.76 | $3.02 | |
| Nov | $7.63 | $13.83 | $7.15 | $7.27 | $6.47 | $4.29 | $3.29 | $3.52 | $3.47 | |
| Dec | $7.98 | $11.18 | $8.32 | $7.20 | $6.89 | $4.49 | $4.27 | $3.36 | $3.70 | |
| Avg | $6.14 | $8.62 | $7.23 | $6.86 | $9.03 | $3.99 | $4.39 | $4.04 | $2.79 | $3.63 |
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