Some people don't like to pay for mods since they already bought the game itself, this is just added content by someone that is a big fan of the game or wants to see extra features, a different approach to the game or otherwise.
It all depends on how big the mod is and how much time the developer has put into his work. Plus the developer of the mod should at least get most of the shares if it is available through steam or any other distribution.
I would not pay for just a small change in gameplay or some small added items/features etc. Perhaps when I start up the mod and I am playing a totally different game with different feel I might consider buying it.
It's hard to say whether or not the mod should be paid or not. For me the best choice would be a donating system. For those that want to support the mod and are enjoying it.
Worempie
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