Dominion Money Purchasing When Actions Lacking
The answer, of course, is it depends. It depends on what's out there and where your deck is going. As a general rule of thumb, buy more money than you think you'll need. It's rare to draw a hand full of silvers mutter a curse under your breath. Don't open not buying treasure or a money-producing card without good reason. 

There's two issues tied up in this: making sure you have enough money to buy more/bigger cards later, and keeping the odds low of "collisions" of terminal actions so you can only use one. Buying a balance of action cards and treasure cards helps with both, but how much to buy depends on the specific action cards you're getting.

I try to create an overall strategy for the particular kingdom deck, and then try to get that deck working as quickly as possible. If you want several $3 or $4 card in your final deck, sure start buying them, but make sure you build the buying power to get the more expensive cards you want. Especially if you'll have extra buys (from Market, Goons, etc.) it's easy to pick up the small cards later. However, without Silver you might not get the big ones in time.

Some illustrative examples:

Village is a terrible buy before you have other actions. The point of village is to play 2 actions after it; if you only play one action after it having a silver would have been better. In most cases, I don't buy Village until I have a couple of terminal actions already. Great Hall is even worse, the only reason to buy a Great Hall early on is if you're going for a combo that it helps (e.g. Conspirator), but even then a first turn silver will probably help you more.

A card like Watchtower, on the other hand, can speed up your opening with it's card draw, and it mitigates the harm from collision with the ability to topdeck new buys, so Watchtower could work with another terminal or with money.

If there's a $5 or $6 card that you want to get ASAP (Witch, Mountebank, Goons), it might make sense to open Silver/Silver, though a Smithy or Watchtower could be just as effective. And an early Gold will help you buy those more expensive actions (and more Gold) for the rest of the game.

Caravan comes to mind as a card that doesn't produce money but I'll usually buy it whenever I hit $4 until there aren't any left.

Chapel is a great card and is most effective when bought before the first shuffle, but it needs to be paired with something that produces money so you can get rid of those coppers fast, which means Silver if nothing else is available. (This one probably has a lot of exceptions, Chapel/$5 attack card could be good, or Chapel/Wharf would be nice.)