Paid mods are DLC but without having to bother to make it themselves. Imagine SkyUI being a priced mod. Heh. Even less incentive to make a game good and proper and work right out of the gate - any annoyances other people can deal with.
Also:
a) One problem with this as "supporting the mod-artists" was they only got 25%. That's fairly ridiculous, depending on type of mod.
b) Donation buttons aren't a good comparison - income, especially on mods, is ridiculously low, to the point of not even paying for hosting.
I can understand and accept paying mods for really big game changers - Counterstrike was originally a mod, after all. Total War: Lord of the Rings? Sure, I'd have been happy to throw them some money, I sure spent enough time in it. Re-colored Skyrim armor - now in blue! €.99! In Black! €.99! In strobing yellow! €.99! ....Not so much.
There's other types of mods, too...Someone makes an original piece of music, puts it up as background in Skyrim. Why does Bethesda deserve part of it? let alone, 45% of it? Even iTunes takes less. Valve, sure, a small percentage for hosting - but for that, 30% is still ridiculous.