Except the Nexus versions of these things are not even close to being in the same league as Dawnguard. I love mods as much as the next guy, but there's just nothing available for Skyrim that's close to a professional level of development.
This.
There are some incredible quest mods out there ('Much Ado about Snow Elves' uses voice actors from the original game, for example, and Moonpath is constantly held up due to the sheer amount of content it adds), but none really reach the quality of professional DLC, and a lot of the shining examples don't integrate brilliantly with the main game itself, feel a bit isolated (exist in their own worlds etc.). The achievements of a mod often get a bit exaggerated because it was made by an amateur rather than professionals - they can be great, yes, but let's be realistic, if a professional company released some of the better mods out there, people would be very critical. Bethesda knows this - it's why they're comfortable charging $20 when the creation kid is readily available.
If you're a PC user, you could also think about it this way: Should I spend 20$ on something that modders already gave me months ago for free? Summonable horse, huge castle, monster follower, vampire/werewolf perks and overhaul, new zone, new dungeon, new monsters, new NPCs and dialogues, new storyline. All of these can be grabbed at Nexus. All of them.
The problem with this topic is that opinion's will differ quite a lot as to what $20 is 'worth' in terms of DLC, particularly when there's so much free content out there. Personally, I'm constantly on the nexus and my game is straining under the weight of my load order, but I don't feel that all that free content necessarily devalues Dawnguard. If there were mods that truly did replicate Dawnguard (or come close) then that would be different - aside from freely available arrow-smithing mods, I don't really see that happening. You might think differently though - playing through some of the excellent mods on the nexus will occupy a lot of time and I can see how that might dissuade you from spending $20 when you've got plenty of free content still to explore. I've run through most of the good stuff, so I feel more inclined to cough up
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