$1:
Sonic the Hedgehog
Sonic Adventure DX
Sonic 3D Blast
Sonic CD

BTA (this looks like it might be one of those where the average ends up being higher than the upper tier):
Sonic the Hedgehog 4 - Episode 1
Sonic & Sega All-Stars Racing
Sonic the Hedgehog 2
Sonic Adventure 2 (includes Battle DLC)
(Mystery games)

$10:
Sonic Lost World
Sonic Generations (includes Casino Nights DLC)
Sonic the Hedgehog 4 - Episode 2
Sonic & All-Stars Racing Transformed (includes Metal Sonic & Out Run and Ryo Hazuki DLC)

$35:
A rather crappy-looking T-shirt, imho.

Free:
Some comics

I own most of what I care about, but I might consider getting this (or some of it) if the keys are handled individually, so I can give away what I already own. Also, bear in mind that if you're only interested in one or two of these, you might be better off waiting for the Steam Summer Sale (apparently due tomorrow), since most of this is bound to be on sale there (looking at you, stupid S&A-SRT's DLC ¬¬). How good (or bad) is Generations anyway?

Don't bother. The keys are bundled by tier.

I never played any Sonic games. Nor do I like arcade games or platformers... Is there anything in this bundle for me?

nope
unless you like mascott racers

I'm mildly interested in the $10 tier, because I'm curious to see if there are any fairly recent Sonic games that aren't awful.
But it's too expensive for that.

the ones in the$10 dollar tier are not awfull
why do you think they are behind that paywall ?

3D blast and the two adventure games are the worst in that line up really

So much this!!! Looks like Sega really doesn't want me to buy their bundles. Well, seeing as how you said you "don't like" platformers rather than "hate them with a passion" and the classic Sonics are among the best in the genre, not to mention historically significant to any video game aficionado, I'd say you at least oughta throw a buck in the hat and try them (Sonic CD alone is worth way more than that, imho). I'd say Sonic 3 and Sonic & Knuckles are very likely to make an appearance as mystery games, which would pretty much give you all the absolute must-haves, but it's not like those are all that hard to come across on their own (and you might be better off getting Sonic Mega Collection Plus off eBay).

Steam keys as far as the eye can see...

or the megadrive collection on the xbox 360 or ps3
40 megadrive games on one disc

Sonic Generations is good.
It's one of the few cases were the devs didn't take a dump and photographed, to use as inspiration for the development of a Sonic game.
And no, I'm not being harsh, just look at that huge pile of shit which is Sonic Boom.