DStu - That's interesting... I'd think that without a way to trash the extra coppers, it would be worse than buying silver since you're getting an average of $1.67 per card ($5 / 3 cards). Same argument I've heard for Hoard not being very good (1 Gold + 1 useless green card = $1.50 per card). But I suppose it also increases variance.
I don't want to look it up, but we had this discusson in some topic some months ago. Think it was about variance and Cache was an heavy example.
The point is, 1.67 is (a little) more than you need on average you need for a Province. And even more than the average/card you usually get to in optimized BigMoney, which is about $7/hand or $1.4 per card.
Silver gets you 1 card with average $2, but Cache gives you 3 cards with average $1.66. Both are about the average, but you add more cards with Cache. You add $5 to your deck in total with Cache, but only $2 with Silver.
Additionally, there is variance, which usually is a good thing, especially if your average money is below $1.6. Cache gives you a $3, and with a little bit of the rest of the hand adds to a Province, if not at least a Gold. Silver gives you a $2, from there it's harder to get up to $8, even to $6.
AND Copper is not that bad of a card for BigMoney, at some point you want to green, the more cards you have the less the green influences your deck.
And finally, Cache it not magic, it's like 55:45 for BM-Margrave (randomly opened in the background at the moment), and maybe 60:40 for others (don't know), so it's not a killer, but it's a slight improvement.