a setup of Hwy/Kc/+buy is pretty easy to get to more VP in a turn from treasure than are in the province pile.
Err... I debate that claim? That would be 48 Buys. 41 if you're generous and assume you still have your starting Coppers. KC'ing all Market Squares leaves you with 31 buys.
Yes, you can either Kc eleven/twelve +buy cards or do any of several really easy things with more specific cards (e.g. Squire, Haggler, Talisman, Beggar, etc.). Assuming your opponent goes off early and tanks on green (e.g. something like buying 4 Provinces, 8 Duchies) you can then overbuild if there are enough +buy. With Kc even a lot of the really crappy +buy count, but two piles with +buy is not exactly uncommon.
6 Bridges is a free province worth of points
Or 6 Bridges is, you know, 3 actual Provinces.
You don't say? And here I thought it was 3 Provinces and 3 Duchies which is even more points.
You will note that I did not say it was two free provinces worth of points, though that is an option, precisely because I assumed you would want all the provinces you can afford.
The point is, that because you are looking at your VP being spread over mutliple cards, the premium for cost reduction is larger. Likewise, once you hit some critical mass of cost reduction, you become +buy limited and on a lot of cost-reduction boards that means you really can score more VP with treasures than with Provinces. E.g. 5 Hwy is around the critical point for making golds +buy limited rather than $ limited; thus a setup of Hwy/Kc/+buy is pretty easy to get to more VP in a turn from treasure than are in the province pile. Getting to the critical point for provinces is around 7 Hwy and that is far more difficult if the pile is remotely contested.
And that is what will make the rare Palace game totally warped - when the marginal value of +buy starts to beat out the marginal value of $. That is when you can Palace for the big stuff.
This seems backwards. If you're buy-limited, copper/silver/gold is expensive because that's 3 buys! It's also 3 stop cards, which is a big deal if you're building a thin engine that can play 5 highways consistently. Finally, it doesn't do a thing to end the game, which is extremely important in a megaturn mirror game. Are you saying this actually happened in a game you played, and that it was the best move?
If you are buy limited on both, then of course, go for provinces unless that is an auto-loss. The point is that is
easier to become buy limited on gold via cost reduction and that buys are, generally, easier to spam in Dominion than cash. Getting enough buys to run straight out for Palace points does take a nice setup, but it is not too bad.
For instance Btroll/Talisman/draw can set you up for 48 VP, anything else that let's you get bonus +buy (like a Ruined market) can let you hit 60 VP. Even a single province + Palace VP is enough to win the megaturn war.
Getting to that sort of +buy count is not common in the least, but for a few setups (e.g. Kc & double +buys) can be doable. And the upper bound of 90 VP is enough that you can let your opponent grab almost all the provinces and all the duchies while you build.
The more common effect is that when you are spamming cantrips or village/X with your cost reduction, you can setup for a respectable number of points (a province every 6 buys) which lets you build for longer - if the other guy goes off too soon you can just build, vacuum up the leftover green, and then punch out with Palace points. Getting to 50%+1 VP is no longer the benchmark for cost-reduction megaturns, it is far to easy to supplement with Palace points to gain 4 Provinces, 6 Duchies, and call it a day.