I had some fun with numbers like I were back at primary school, as I am always skeptic about DLC for the nature of what DLC stands for. Hopefully I didn't fuck up with the number punching, that would be pretty embarassing. Long post, worded and formatted horribly, so be careful.
The cost of each single DLC is £7, roughly 1/7th of the full game. Let's avoid the bundle discount, as that would include things like buying the game second-hand, obtaining a copy from the shady eastern european "shops", buying the WiiU bundle, or using premium membership deals, whatever a shop may have for you, to balance things out. We are going to stick with the individual suggested price because it would be hell to consider all those.
One DLC contains 3 characters, 8 tracks, 4 vehicles (which I am assuming it's 4 parts of body, tires, and gliders?), totaling 12 parts
Characters in the base game: 36
Tracks in the base game: 32
Vehicle parts: 26 bodies, 18 tires, 12 gliders, total of 56
Let's see what one seventh of the full game contains:
Characters: 36/7 = 5 rounded down
Tracks: 32/7 = 4.5
Vehicle parts: 56/7 = 8
Pretty fair comparison all in all, right? The tracks are by far the most demanding part of the content, and we get almost double that. The thing is, this isn't fair at all.
Now, considering the full game comes in with the attached price of the full development process, including the engine, the optimization, the marketing, possibly the development of new tools to facilitate the development, the manufacturing, the shipping, the retail cut of the physical copies, I think it's extremely generous to assume that 25% of the final cost of the game went into developing the assets themselves (modeling, texturing work, optimization, track design, playtesting, balancing), most likely much, much less, considering this isn't, say, Gran Turismo, a serie with hundreds of licensed cars - but let's go with this anyway.
We then use the estimated end-user price of the game that is responsible to cover up for the assets cost (again, extremely generous assumption against my original argument, because we do not know the production costs), which is essentially what the DLC is made of entirely, for a fairer comparison:
Cost of the assets within the base game: 50/4 = £12.5
Cost of a single DLC: £7
Here's what we get if we divide each individual asset category (character, track, vehicle part) by the above values:
BASE GAME ASSETS
Individual character value 12.5/36 = £0,35
Individual track value 12.5/32 = £0,39
Individual vehicle part value 12.5/56 = £0,22
DLC ASSETS
Individual character value 7/3 = £2,33
Individual track value 7/8 = £0,88
Individual vehicle part value 7/12 = £0,58
This, however, is not still not fair at all. We are dividing all these categories equally, when tracks, of which the DLC has a much higher ratio of, compared to the base game, are worth way more effort than a vehicle part. This puts my assumed DLC value at a big disadvantage. As for characters, there is animating and voice acting, the latter being very expensive. An entire track should still be more costly overall than a character, so let's modify the above values to accomodate for this:
Vehicle part value: 1
Track value: 200
Character value: 50
This is my estimated value of all the assets' categories related to each other. It's very possible that characters would rank much higher due to voice acting, but once again, I will keep it down to try and disprove my original argument, since the DLC is most lacking in those.
Applying this to both the base game assets value, and the DLC, we get this:
BASE GAME ASSETS
Characters: 36*50 = 1800
Tracks: 32*200 = 6400
Vehicle parts: 56*1 = 56
TOTAL VALUE: 8256
FINAL VALUE/PRICE RATIO: 8256/12.5 = 660.5
DLC ASSETS
Characters: 3*50 = 150
Tracks: 8*200 = 1600
Vehicle parts: 12*1 = 12
TOTAL VALUE/PRICE RATIO: 1762/7 = 251.7
Finally, by dividing those two numbers, the value of the content from the DLC is worth 2.6 times more than the value of the content from the base game according to my pulled-out-of-the-ass estimates. And remember, this is assuming an overly bloated asset cost compared to the whole production process, an assumed low cost of producing a new character, and most important of all, I assumed a £50 base game price when the majority of the big sellers are going lower (for example amazon.co.uk has the game new at £37, which is considerably lower). All of which are intentionally giving more final value to the DLC for the sake of my assumptions being just assumptions. So you can pretty much guarantee my estimate to be varying degrees of generous in favor of the DLC.
I suppose that we also need to attribute a small portion of the production costs for the marketing of the DLC itself, putting up the updates on the website, digitally delivering the update, and making statements. However, if we compare this with the dozens and dozens updates, lengthy developers interviews regarding balancing/features/design, conferences, and marketing of the base game, I think it's pretty safe to say it's comparable to a drop in the ocean.
In short, I don't find the pricing fair at all. I think people are confusing "less scammy than others" as "fair pricing", which are two very different things. This DLC, like every other DLC ever made, exists for one purpose: making mad dosh. DLC exists because adding assets to an already existing game costs much less to finalize, while the userbase is fine with retaining a cost/effort value similar to that of the base game, despite there being massive unfair differences with the two cases, which promptly happily get ignored by the masses.
tl;dr
I hate DLC. Keep that new work for the next version of the game, even a standalone expansion, which is properly updated. It's ALWAYS better in terms of cost efficiency and content efficiency for the end user in the long run. It's a shame that nintendo has been walking this path as well lately, but at the same time it was inevitable because it's that much profitable. And as per usual, the people voted that they don't mind smaller amounts of content injections for a bloated pricing, so it's hardly ever the company's fault for being "evil" in these cases. Oh well.