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| | 1999 | 2000 | 2001 | 2002 | 2003 | 2004 | 2005 | 2006 | 2007 | 2008 |
| Jan | $1.77 | $2.34 | $9.98 | $2.56 | $4.99 | $6.15 | $6.21 | $11.43 | $5.84 | $7.17 |
| Feb | $1.81 | $2.61 | $6.29 | $2.01 | $5.66 | $5.78 | $6.29 | $8.40 | $6.92 | $8.00 |
| Mar | $1.67 | $2.60 | $5.00 | $2.39 | $9.13 | $5.15 | $6.30 | $7.11 | $7.55 | $8.93 |
| Apr | $1.85 | $2.90 | $5.38 | $3.47 | $5.15 | $5.37 | $7.32 | $7.23 | $7.56 | $9.58 |
| May | $2.35 | $3.09 | $4.89 | $3.32 | $5.12 | $5.94 | $6.75 | $7.20 | $7.51 | $11.28 |
| Jun | $2.23 | $4.41 | $3.74 | $3.42 | $5.95 | $6.68 | $6.12 | $5.93 | $7.59 | $11.92 |
| Jul | $2.26 | $4.37 | $3.18 | $3.28 | $5.29 | $6.14 | $6.98 | $5.89 | $6.93 | $13.11 |
| Aug | $2.60 | $3.82 | $3.17 | $2.98 | $4.69 | $6.05 | $7.65 | $7.04 | $6.11 | $9.22 |
| Sep | $2.91 | $4.62 | $2.30 | $3.29 | $4.93 | $5.08 | $10.85 | $6.82 | $5.43 | $8.39 |
| Oct | $2.56 | $5.31 | $1.83 | $3.69 | $4.43 | $5.72 | $13.91 | $4.20 | $6.42 | $7.47 |
| Nov | $3.09 | $4.54 | $3.20 | $4.13 | $4.46 | $7.63 | $13.83 | $7.15 | $7.27 | $6.47 |
| Dec | $2.12 | $6.02 | $2.32 | $4.14 | $4.86 | $7.98 | $11.18 | $8.32 | $7.20 | $6.89 |
| Avg | $2.27 | $3.89 | $4.27 | $3.22 | $5.39 | $6.14 | $8.62 | $7.23 | $6.86 | $9.03 |
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