Making a quick plugin that takes 5 minutes to code like no PvP while in fly mode or PvP Toggle shouldn't charge anything, if $5. However a full fledged custom minigame or plugin as complex as Factions or Economy could run $500+.

It's not hard to learn how to code. Just think of this way. It's how I learned taking college courses. Imagine what you want to do, and put it in a flow chart. Download a flowchart software to actually visualize it. Start from the beginning. Say you want to block PvP while flying. Start with "Is the player flying?". Then there's really only two things left. If yes, disable PvP, if no, do nothing. It can be as simple as that or a clutter of if this, if that, when, etc. It all depends. As a programmer, making flow charts of your program can make the process 100% easier. The rest is just understanding how it is put together in Java syntax and loading the proper libraries, configs, and files.