Broadly speaking the two are orthogonal. That is, financial sustainability isn't related directly or consistently to environmental sustainability. Specifically, there are examples that swing strongly in both directions.
At one extreme, you have subsistence farmers or hunter-gatherers who don't use value-based exchange at all, and different groups of those people have environmental impact ranging from barely perceptible to devastating (the widespread megafauna extinctions as humans arrived, for example, or the expansion of deserts through grazing and firewood gathering).
At a fully monetary level you have profitable companies who build nuclear weapons (one extreme of the "environmental impact" spectrum) and similarly profitable companies who revegetate artificial deserts.
If you're embedded in a market-based society (as most of us are), the theory behind that is that given a complex set of requirement, a market is the best way to find some optimum solutions (specifically, that there will probably be more than one depending on how people weight different factors). So for a single product or commodity, as long as the production mechanisms are equivalent price is an excellent way to select (that's the justification for a market economy).
Unfortunately it's extremely unlikely that "all things are equal". It's trivial to buy financially unsustainable things in most countries (made by slaves or prisoners, for example), but deciding that you won't buy slave products is effectively impossible. Either because there are no competing products (cellphones, for example) or because the manufacturers try very hard to conceal that problem (clothing).
Environmentally the problem is the same. Even if you just consider superficial impacts, the cost of biofuels varies dramatically depending on apparently trivial things. Consider corn-based ethanol, since that's common, and think about trying to buy it if you want to avoid, say, pesticide resistance. Some corn farmers will grow the required pest reservoirs and use pesticides as suggested, but others won't. Unfortunately there's no auditing of that, and definitely no labelling at a retail level. And that is only considering one tiny aspect of what makes a single biofuel sustainable. Realistically your "sustainability choice" becomes "does this seem less unsustainable than that?"
At a societal level the way that externalities are monetised is critical. Viz, pollution and other unsustainable activities must have a price attached to them, either through a tax, regulation, a counter-subsidy, or a ban. So we ban slavery, tax carbon emissions, regulate pesticide use and subsidise green electricity. All of those things work to a greater or lesser extent, and which option is chosen is often as much politics (political or social sustainability) as science (ecological sustainability).
At a personal level, you have to decide how much effort you're willing to put into discovery, and then again into minimisation. Economic theory tends to assume that everyone has perfect, complete knowledge of everything (with the exception of anything using terms like "discovery costs"). The simplest cases are where there's only one or two options. With cellphones it's often "my friends all have iPhones, I'll buy an iPhone. They're all using WobblyTel, I'll use them so I get the free Wobbly2Wobbly messaging". Your choice then is between a new iPhone and a second hand one. Environmentally that's an easy choice.
For anything more complex the question is how much work you're willing to put in researching every aspect of everything you consume. To some extent you can rely on secondary sources (other people's research) and tertiary sources (summaries and guides assembled by non-researchers), which is where things like the Greenpeace shopping guides come in. But to have any confidence in those you need to do more research... it gets ugly.
Then when it comes to buying things, you need to decide whether you're willing to (say) travel across town to buy a better product rather than just buying whatever is closest. In other words, even the "cost" of an item is more than just the money you hand over to the retailer.
One common solution that's fairly effective is to shop at a local co-op, commonly a food co-op. That way someone else does the research, and you go to one place for most of your food shopping. Simple and direct. Often you can do similar things with other purchases, or rely on certifications, but usually within a fairly narrow range. There's no "fair trade smartphone", for example.