Hey there!
You mention a ball park number for a RTS game to be used within your NFT collection, needs to be fun, and have a progression over time.
NFT's can be implemented in your game, but I cannot give a ballpark number without knowing how lengthy is your game, what else will be included, user accounts, you mentioned skin, also some sort of leader board, and this of course, including multiplayer.
It can starts from $10,000 and UP. (again, depending on how complex is your game)