One should look to Skyrim for example on how a modding community is sustained. It is still one of the most played games on Steam purely due to mods. And the modding tools are completely free. Any price attached to being able to mod is a barrier to entry, basically a paywall. I do not support such ways. There should be a way of giving tools for free without spending a year writing them or making your customers pay. Modders also do not need source, they are modding your game instead of writing a new one on top of yours.