Perhaps that is a difference in deckbuilding philosophy then? In terms of the subpar cards argument anyway. I looked at 3 of my decks and could not find a single subpar card within them. And anything that could be argued as subpar is likely there as a testing slot. Or perhaps some of the foundation making the deck work. Almost every set I have to go through my decks and cut cards I like just to make room for new ones. For example, within my last three rounds of changes for Karador I cut Erebos, Mikaeus, Protean Hulk, Birthing Pod, Greater Good, Kozilek, and Oracle of Mul Daya. Among other cards of course. These can hardly be considered subpar cards and yet they still made it to the chopping block because they simply weren't what the deck needed anymore.
The impetus for doing so of course was to play with new toys: Luminous Broodmoth, Mythos of Nethroi, Eerie Ultimatum, Fiend Artisan, Elder Gargaroth, Tevesh Szat, and Magus of the Order. Again, among others. While the changes may not stick, it is hard to argue that the deck contains subpar cards simply because the deck size is 100 cards. In fact, I have often seen the opposite argument: the deck restriction should be a minimum like it is in other formats so people can play with 101 or 102 cards as ways to test out new additions.
As I said, still disagree with the cost aspect. In Karador alone, I have 21 cards (not counting lands) under $1. These represent some pretty good cards in the deck but even if I simply said these were the worst, based on cost, they only come out to be $9.41. If I just take out the 15 lowest cost cards, since the other 5 would be lands, the cost is $4.23. So even accepting that the deck has "filler", the cost savings is relatively minimal. Do note, these 15 include things like Sun Titan, Crop Rotation, Gray Merchant of Asphodel, Command Tower, Magus of the Order (I will concede this could end up being subpar, but it is being tested), Wall of Blossoms, and Unexpectedly Absent.
So, while our quick and dirty math shows an obvious difference in cost between 100 and 80 cards, I still maintain that if you are running subpar cards, your cost in doing so in negligible. Subpar cards are not $1. *Good* cards can be had for under $1. And even if you are not running subpar cards, the costs are still reasonable.
I do understand your point: fewer cards means less cost. Easy. Simple. And I realize your "$1 for bad; $2 for good" is just illustrative. But the cost isn't where it is because you are filling the deck with subpar cards because you shouldn't be using subpar cards to begin with. The cost is there simply by virtue of needing more cards. Those additional cards can easily be good and, somewhat easily, be done on a budget.
b) Shuffling 100 cards isn't fun. And that is why most play the game, for fun. Do you really enjoy shuffling 100 cards? While its possible to build a fetching/tutoring deck, is it really fun to shuffle 100 cards every turn?
I might be in the minority here, but I *do* prefer shuffling 100 cards (and I double sleeve mine and I have "normal" sized hands). Whenever I shuffle a 60 card deck, it just feels like I have nothing in hand. I have no problems with the shuffling after every search and I have no problems shuffling the deck in general.
c) It costs more. I love to spend money, heck who doesn't. But who really likes to find and then spend hundreds (or even thousands) to test a new possible mechanic or to build a top competitive 100 card commander deck?
d) It can force you to add subpar or weak cards. Having 100 cards can sometimes force you to add weak cards for that mechanic simply to fulfill the 100 card requirement. Do you really like to add and then draw weak cards?
These two things are at odds with one another. It can't cost more *and* force subpar cards. At least, they can't both be used as generalities. In any case, I disagree with point 'd' (I don't really agree with point 'c' either but I have a feeling that is more subjective). There are 20,000+ cards in Magic. If you are running subpar cards either you didn't look hard enough or you think anything worse than Cyclonic Rift or Sol Ring is subpar. They may be "worse" but there are plenty of good cards you can include in decks.