I make custom Minecraft plugins for Spigot or Bukkit servers. It may work on other server types like cauldron, but it is not definite.
Small simple plugins like custom items that do cool things, modifications to game mechanics, or anything fairly small will cost around $5 - $20. As the plugins get bigger the price will go up.
I will give only you the plugin and I will not post it for anyone else. This is only for Bukkit/Spigot Minecraft servers, not Forge or any other game. If you want the plugin to interact/change another plugin (like a store that uses money from another plugin) it will cost a bit more.
I will provide free bug-fixes for up to two weeks after I deliver the final product. After that if you get a bug or want something added, you will have to pay to have it done.
Time: Depending on how big the plugin is, it may take longer to deliver it. A small plugin will usually take 3 - 6 days, but bigger ones may span to 1 - 3 weeks.
Note: This is a plugin, not a mod, so I can not add any new graphics or sounds into the game. I can just use and change stuff that already exists in Minecraft.