Okay, so, I just wrote 8000 characters for this topic and closed it without saving, go me. But, I feel that this topic is so important that I am going to write it all again. It will probably be shorter this time.
First off let me say, I think modders SHOULD be paid for their work. Because development for a mod IS WORK.
Secondly, I am basing most of my post under the assumption that some mods will be very high quality.
Thirdly, I understand that there is a lot about large companies that I do not understand, so, I will stay away from price as much as possible. The only thing i'll say about price is that the mod creators should be making a larger percent than they are now. I am aware that Valve is making thirty percent and letting the game creator decide. There needs to be a way for the content creators to be ensured at least 33%. With three parties in action, Valve, the game developer, and the content creator. The content creator should get the largest portion of the sales.
On to Pro's
Content creators will get paid for their work. This is good. People getting paid produces a larger commitment on the part of the content creator. Plus, despite what anyone says, the content creators should be getting paid for this. Or, they should at least have the option. Donation buttons almost never work because there is no incentive for the average person to ever donate. They will get the mod either way, where is the average person's incentive to donate?
The developers may be more inclined to make development kits like Bethesda has done for their games. This makes it easier for modders of all experience levels to create mods.
Valve has a way to get paid directly through the use of their servers. Valve is the one hosting the files, Valve is the one providing the interface in which people download the mods, they should be paid for it if people are making money with their help.
We may see professionals joining the modding community in order to supplement their income. This means we could see a rise in the quantity of extremely high quality mods. (not that there aren't already.)
We will see more frequent updates on more popular mods so authors can insure that their stream of revenue will continue to grow. Hopefully this will include tons of compatibility updates.