Mob Bounty Reloaded is a plugin that adds cash drops to monsters! As will most economy plugins, you need Vault and an economy plugin to make it work. Mob Bounty Reloaded is multiworld compatible, supports both PvE and PvP, has highly configurable rewards, including negative rewards to fine for kills, supports kill streaks, has language localization, and has many multipliers for drops such as time, world biome, permission group, and more! That's a lot of features!
Development builds have not been approved by the BukkitDev staff. Use them at your own risk!
See The Plugin In Action! AbsintoJ made a tutorial video on the plugin (it's not in English, however)