William Brock told investigators he shot Loletha Hall because he thought she was working with a man who called and threatened him and his family. Hall didn't threaten Brock and didn't have a weapon or assault him, according to the sheriff’s office and court documents.
I really hope that was a typo and they meant AND a payment of $15k because someone who posted $200k bail shouldn't have a problem finding $15k to get out of a murder charge.
So yes a typo. The typo they are referring to isn't the amount. It's the fact that the article says the fine can stand in place of the prison sentence. Your link specifies the fine is "In addition" which means they get jail AND a fine.
Because it was set many decades ago and never adjusted for inflation.
(I haven't bothered to check if that's true for this specific case, but it's a pervasive problem throughout the US legal code so I'm hazarding a guess.)