Quarry & Ferry are obvious choices for stuff that likes high cost more than low cost, but pretty much all the cost reducers scale geometrically with the cost of useful cards on the board. I am much more likely to build out to $7. Hop was also a big winning on expensive boards.
I would have thought the exact opposite. If a card is so expensive that you can only afford 1 of them, then Quarry is just a Gold (a Gold is pretty good, but still just a Gold). Whereas if there's cheap cards that you want a lot of, then Quarry makes it much easier to but a bunch of them, quite possibly even 1 for every buy you have. Basically, with all cost reducers, they get better the more cards you buy per turn. Expensive cards are harder to buy multiples of, even with cost reducers.
Timing. Ferry on a $5 means you can open a power card. Ferry/Quarry on a $6 (e.g. Altar, Artisan) or $7 (e.g. Forge) can knock whole shuffles off your build out time. Using a $4 to get a card on T3/T4 that you normally would not see until T7/T8 is often a 12 VP swing.
Additionally, playing around with things like Ferry and Quarry come in direct conflict with just buying the cheap thing you want anyways. Say the clutch card is $3. I drop a Ferry on it. If it does not come with a +buy itself I need a +buy to use my Ferry at all. This means that on T3/4 at best I end up with 3 copies of the card in my deck. My opponent most likely gets four just buying them every turn. At best then, I get my fourth and fifth copies while my opponent gains his fifth. I come out whatever the +buy is ahead at the risk of increased risk of losing the split very badly (e.g. my second shot of +buy comes later in the shuffle). $4 cards are even more dicey as I need to line up +buy and enough cash to use it.
We have another card that is phenomenal at getting many copies of cheap things. It is called Talisman and while I think it is undervalued in the main, it is nowhere near as versatile as say Quarry.
Ferry is not as good on lost cost boards as it only hits one pile at a go, typically low cost bulk is going to be a mix of draw, village, payload, attacks, thinning, etc. This means you burn several buys on moving the Ferry or only using the cost discount something like 5 times.
Quarry is better at the game you propose, but it is yet much better doing the same thing on a high cost board. Quarries stack and suddenly Quarry/Quarry/Market/Copper gains a Kc/Market turn 1, then 2 Kc & 1 Market and then you can pile all the markets the next turn.
So basically I look at it like this:
Cost reduction for "singleton" power cards (e.g. Altar, Forge, Expand) make Ferry/Quarry that much better because getting the big guns out on T3/T4 is so huge.
Cost reduction for "massed" power cards (e.g. Minion, GMarket, Kc, Nobles, Hparties, Possession) means that you can either hit them
every turn with Ferry or stack them with +buy for mass gain with Quarry.
Obviously, you are completely correct that these cards let you bulk up on cheap stuff easy (e.g Ferry/Steward is an excellent opening for massing something like Caravan), but it is fairly rare that big cards both time insensitive AND not getting massed (often being the the key split to win). Of the times when that rarity occurs the big cards are most likely something you are ignoring ... and quite likely would ignore Ferry/Quarry as well for gaining cheap stuff (e.g. Silver flooding can make all cost reduction suck).
So no, I would totally stand by Quarry and Ferry working better on high priced things. Shaving off a shuffle is huge while being able to buy $5 on average starting hands/stacking +buy & cost reduction is way too powerful. Quarry's power grows linearly with each +buy you utilize (which is awesome on cheap boards), but grows grows mildly exponentially on high cost boards with +buy.