I still completely disagree with that. Once you have 5 silvers in your deck you do not suddenly start buying wishing wells, lookouts, embargoes, pearl divers, chancellors, thieves, caravans, mines or in fact any alternative card at all. If you want something else than silver then you should generally get it very early before the silver fills the deck. If you are using a lot of silver then keep buying silver. Just what 'any action you could purchase' are you talking about?
The basic premise is that, by the time you have 5 silvers in your deck, your money density will be around $1.5 per card. So any card that offers +1 card automatically offers +$1.5. (You already have a 45-50% chance of replacing the silver with that +1 card).
The second basic premise is that we're operating under the assumption that you haven't already maxed yourself out on terminal cards and can add another terminal.
The third basic premise is that
there are most likely alternatives to any crappy card you mention. By the time you have 5 silvers in your deck, you only have a ~5% chance of drawing a hand with
exactly 3 coins. And, there is only a ~5% chance that the kingdom has
exactly one <=$3 card. So in 1 out of 400 hands, you'll be faced with a dilemma of "Silver vs. One Specific Crappy $3 Card". The odds are even less for "Silver vs. One specific crappy $4 card".
Wishing Well: It's a cantrip, so $1.5+ Utility. If you haven't done any trashing, you have between a 30-40% chance of hitting Copper if you name it, which adds $.3 to $.4 to its value. So, overall, Wishing Well is worth on average $1.85 per play. So strictly speaking, it's
slightly less valuable, $-wise, than silver. But it provides two key utilities: if you
do hit Copper, you have filtered a copper out of your next hand which increases your next hand's value.
And you have the ability to do the hail-mary Wish (if you have 5 coins and you need one province to win, WW will always be more useful than silver). .
Embargo: $2+, Utility, thus more valuable than silver. (As long as it doesn't collide).
Pearl Diver: Cantrip, so $1.5+ Utility. The utility is fairly marginal. You have a ~50% chance of hitting a >=$2 card on the bottom of your deck, thus increasing your next hand's value by ~$.7. So that's ~$.35 worth of utility making it worth ~$1.85. It's highly situational, but yes I would agree that in the 0.25% chance that this is the only <=$3 available, it would likely be preferable to buy silver.
Chancellor: $2 + Utility, thus more valuable than silver (as long as it doesn't collide).
Lookout: The filtering increases the value of your next hand by ~$1, and the trashing and increases the value of all future hands by ~$.20. So it pays for itself in 5 turns. Probably not better than silver, but board-dependent.
Caravan: Cantrip so +$1.5, provides another cantrip so another +$1.5, so $3 of total value.
Thief: Except in extremely specific situations, less useful than silver. But, that's counterbalanced by the fact that there's approximately a .01% (1 in 10000) that you will have
exactly four coins, and that this will be the
only <=$4 card available.
Mine: Provides +$1 in value, plus trashing, (the result of which increases the average value of your future hands by $.25). So in four turns it pays for itself. Probably not better than silver.
To answer your question re: "What any action you could purchase?"
1. Any card that provides +$2 (Worth $2+ utility)
2. Any card that provides 2 or more cards, regardless of whether or not there's a discard involved. (Worth $3+ on average)
3. Any card that provides $1 + 1 card. (Worth $2.5 + utility on average.)
Again, let's all take a step back and remember that this is for the
average scenario. Edge cases need not apply. There are definitely individual cards that in almost all circumstances are worse than silver, but the odds of that one card being the only available option range from extremely low to astronomically low.