In most society, it is considered acceptable to ask for retribution in exchange for a service or a product.
The motivation inciting a modder to produce a mod can range from just for himself to intending on distributing it for profit. There is no bad reason to make a mod and the reason is no guarantee of the level of quality of the mod. Talent, work, passion and perseverance will produce great mods.
Expectations are usually higher if the mod as a price tag, but in my experience the best mods are not behind a pay wall but from passionate and talented individuals doing it predominately for them self, proud craftsman taking reward in creating an accurate replica of something they admire. Once their creation has reached a level they can be proud of, they may or may not share it with a little group or the whole community.
We should be great full to any modder, rookies or masters, sharing the fruit of their labor with us, as a gift or as a product, they all deserve our admiration, it is up to us to decide if it is worth paying or donating.