According to Dave Manuel's "Inflation Calculator" website [1], $500 in 1864 equals nearly $7,700 today. The monthly pay for a private in the Federal Army during this time was $13 ($200 in 2016 dollars); it would rise to $16 ($246 in 2016) in June 1864.
History of 1st Illinois Light Artillery, Co. E The Order of Battle for the Battle of Westport does not list this Battery, so it appears doubtful that they took part in this, the decisive battle of Price's doomed campaign.