I have seen many people fight this out over the past couple days, and I am going to voice an opinion from a mod developer.
I do not currently work on any mods due to school and work(c# programmer). (Also to note when I created mods steam work shop did not exist.)
Here is exactly why paying for mods should not be a thing on steam.
Hinders the modding community.

• Modder’s including me use other Modder’s notes, ideas, code, and help to build with them or off of them. Adding this monetary value hinders friendly help from fellow Modder’s do to code selfishness, not saying there is anything particularly wrong with that. But if you are truly a Modder it destroys what the point of nodding is. Amateurs following a dream and creating something for your favorite game for everyone to enjoy, no matter the income they have.
  
  
• people will turn against others to make their personal gain more lucrative, (nothing wrong with capitalism, but I think you should stop working on mods and create your own game.)
  
  
• Stealing, already done on steam! People stealing others things. Specially art. Most games have open art. Meaning no special encoding just png or jpg. IE anybody can copy and paste. What if I take a free mod and copy the weapons and sell them? Like for $0.50 a sword. Yes Stealing has already happened on steam, to make a quick buck. Thanks to the fact mods are so open…

It is a cash grab and undermines the Modder. (Honestly biggest reason not to support steam pay for mods)

• So how much of a percentage is being taken from the modder? 75% - 90%? Something f***ing Ludacris. THERE IS NO VALID REASON TO TAKE THIS MUCH! None! I know all angles of a software development firm. People will say mod developers should pay for what? What tell me? Have I used tools to create mods yes! Guess what all professionals use tools to do their work on a computer. Media designers use Photoshop or Gimp do not need to pay a percentage of their finished product. Programmers use Visual studio or (freeware ie. CodeBlocks)and Microsoft does take a % of finished product, Some modders use non-steam tools or freeware tool s, or none at all depending on game. IE. modding for Terraria, which is the last game I modded I used another Modder’s tools to create “C” programming to mod Terraria.
  
  
• Steam does not need to charge any money to for the mod? Why what is steam losing? Honestly? Arguably storage? Network bandwidth? Server maintenance?
  OK cool how much do those things cost?
  Storage is cheap that why MEGA offers 50 Gigabytes free.
  Network band cheap, YouTube, Facebook free.(pays with ads)
  Server maintenance is cheap, every website has server(s)(some sites like Wikipedia charge NOTHING, Solely by donation).
  For steam it’s cheap, how? They are a service website that sells games and takes a cut of all games sold.
  If steam takes a cut of games shouldn’t Modder’s have a cut taken? It’s not the same for a mod, mod development is usually not someone’s job.
  But if you really want to make someone pay for the mod they created how much should you charge?
  To pay for such a small foot print as a mod on the workshop it most likely cost about $0.05 cents a month? But want to charge 75% per mod sale? That’s f**ked.
  
  
• Should Companies charge for modder’s to make mods? Considering mods keep your game alive many more years past its prime, and generate so much for revenue than your game would have made without them. IE minecraft, the answer is NO!
  
  
• having such a high cut taken from them on purchase, is like saying bake a cake ill take 8/10 and give you 2/10 and look at all the food you have! I honestly think the only thing worse than piracy is this. It’s like saying thanks for your time and effort and hard work! Did you want to get eat food this month? Well to bad, stop being entitled.

Entitlement, No your mod is not that good.(*****NOTE***** I am not bashing Modder’s some a great and spend so much time, making their dream, their idea, their art perfect!)

• Some Modder’s do not support their mod indefinitely.(for other reason than not enough money).
  
• If there is a glitch or bug it might not ever be fixed. Ever
  
• What if a game update breaks the mod? Most of the time game updates break mods! Specially Minecraft updates. So now what if you paid $10 dollars for a mod that never is patched to current updated version? You’re out of your money!
  
• I expect the same high level of integrity that I give others when I charge for a program I have created. So it better work for as long as the game exist or is patched. It better be thoroughly tested and bug/glitch patched.
  

Tldr: DONATE to MODDERS of Mods you like, Don’t destroy the modding community, Don’t buy into Steam money grubbing cut for mods! SUPPORT Modder’s But not like this Steam not like this….