Stonemason is not really Tier 1 if you're actually paying $2 for it though. It almost should be compared more to $6+ cards
Stonemason is a $2 card. For $2, you get the Stonemason card along with a sweet deal on other Action cards. That sweet deal is part of the Stonemason card. The extra money you're paying isn't for Stonemason, it's just money spent in conjunction with what is essentially an on-buy effect to gain cards more easily than you otherwise could. True, it doesn't directly compete with $2 cards, but it doesn't really directly compete with anything given its unique gaining ability, so we might as well rank it with the $2's.
You're not telling me anything I don't know--my point was that for its on-play ability alone I consider it far from Tier 1.
So why are you only considering its on-play ability? Every other card is ranked based on its on-gain, on-buy, on-trash, etc. abilities. Inn would be a lot worse if not for its on-gain ability; Farmland would be strictly worse than a Duchy if not for its on-buy effect. These cards are priced to include those effects, you should consider them in the rankings.
Because you pay $5 for Inn together with its on-gain ability. You pay $6 for Farmland, and perhaps more relevantly for Border Village, with its on-gain ability. You most often pay $7 for Stonemason with its on-gain ability.
I'm mostly playing devil's advocate here, but I think it's an interesting question. If you're going to rank it against the other $2s, you maybe should only be considering the part that you can actually get for $2, which is not an elite ability.
Someone better than I am can maybe argue that I'm underestimating Stonemason's on-play effect, which is possible, but for me it often becomes a dead card.
Stonemason's on-play ability is extremely useful. I agree that it would be a pretty weak card if that were all there was to it, but it's not. I just think it's misleading to rank only the overpay cards based on their on-play effects alone when we take on-gain and on-buy effects into account for everything else. I agree that they present a bit of a conundrum for ranking purposes since you pay so many different amounts for the same card, but at the end of the day it's still essentially an on-buy effect, and as such should be factored into the rankings. I see your issue with ranking it with the $2's, but I don't think it makes any more sense to rank it with other costs, and card rankings will ultimately be flawed no matter what system we use. I just think completely ignoring a component of the card, especially a major component that is usually the card's main purpose, is a bad system.
I think that ranking cards by cost has become less and less significant.
I'd love to see the result of a collective tierization of the cards from the f.ds for. This would be regardless of cost, and mostly based on how deeply their presence affects your strategy.
Tiers could be something like:
F: you could remove this from the kingdom, I'd play exactly the same 99% of the time (eg: Scout)
C: Weakish card, can be useful and even shine, but rarely. (eg: Pirate Ship)
B: Decent card, can be useful and even shine in the right conditions (eg: Village)
A: Strong card, defines a Kingdom and good players will have to consider seriously whether they really want to pass on this. (eg: Witch)
Ü: Almost always a must but, need an excellent reason to pass on this (eg: Rebuild)
This would also have to take into account how some cards (like Thief and Possession) shape the players' strategies even without ever being bought.
(just a random thought, since OP got answered already and the reign of Off Topic has thus begun)