[from the Forums of the Libertarian Left]
Actually, there is no problem determining costs without prices -- assuming that you don't insist on having your costs represented as prices. Now, it is not clear that one can compare kinds of costliness against one another in a non-market economy with the same apparent ease as we do in a market economy -- but, again, there are good reasons to suspect that those cost comparisons are far from bulletproof.
Imagine a biocentric society, which emphasized interconnectedness over individuality, as well as interconnectedness with "the environment" -- an ecological communitarianism. It has its own notions of property, which are heavy on recognition of collective property rising from the associated labor or specific collectives, and acknowledge overlapping property claims. It sets the bar for resource appropriation very high, outside of a well-developed ethic of seeing to one another's basic needs. But it also prizes singularity, which provides at once an alternate support for individual freedom and a rough and ready yardstick for measuring the cost of the loss of a species, or ecosystem, or view, etc. Based on these values, the practical economic details might work out in a variety of ways, but let's make our ecological communitarians broadly mutualist in their political economy. Coming to different conclusions about individual property than most societies, they still opt for equitable commerce between singular individuals, for reasons that are consequentialist or dependent on an antinomic principle. And they believe firmly that the exercise of individuality should be at the individuals' own costs. There's nothing here that couldn't be teased out of the writings of Proudhon, Leroux, Warren and Ingalls, with the addition of some more contemporary science. And there's nothing that couldn't be derived from a different reading of the same data treated by the natural rights tradition. The values are even values that we give more than a bit of lip service to.
If you start with this society, which significantly does not start with a notion that all costs can be compared to one another on a dollars-and-sense scale, a couple of things seem likely. Ecological awareness is likely to give the "ongoing projects" of nature some virtual standing in human decision-making -- particularly because we are just as unable to calculate the impact of natural-resource appropriation/ecosystem disruption as we are to make other sorts of large scale calculations -- and because a strong sense of the various kinds of interconnectedness makes those appropriations/disruptions much more likely to be understood as invasions (of a variety of sorts.) At some point, even if the individuals involved had experience of the process of reducing all values to exchange-values for comparison, other considerations are likely to lead them to consider many cost-comparisons impossible because the kinds of costliness are simply incommensurable. Strip mine (and its resulting ecosystemic alterations) vs. mountaintop (and its role in the current ecosystemic systems) -- between the complexities and the very kinds of costliness involved, it's hard to see how reducing things to a comparison of "prices" (prices for what, offered by who to whom, with what attendant cost, etc., etc., etc.?) is likely to be anything but a travesty. The material complexities alone suggest this, and when the subjective elements are included (subjective values and subjective costs) the notion that some market-clearing price (and the ability to raise it, outside of any inclusive deliberation between the interested parties) could "decide" the issues just seems silly. Certain kinds of things wouldn't get built in this society, but other things would be logical outcomes of the different set of property rules, notions of profit, etc. My guess is that the society in question would be more socialistic in general assumptions, but better avoid the kinds of to-the-highest-bidder evasions of self-management and self-government that markets are presently so good at.