Please note, I'm not ranting at all, and I'm not against paying; I would like to understand the rationale behind setting the prices as such, especially knowing that ISO is network of national standard institutes (i.e. funded by governments). And I also doubt that these prices would generate enough income to fund an organization like that, so there must be another reason.
With regards specifically to ISO standards, there is a question/answer in their FAQ that addresses why ISO standards cost money:
ISO standards cost money to develop, publish and distribute. Someone has to pay. The current system whereby users are requested to pay for the standards they use, not only sustains the development process but also, very importantly, ensures that the balance of independent vs. government, private vs. public interests can be maintained.
With regards to other standards organizations, I suspect that Yannis Rizos is correct in his comment and that it is similar to the ISO's stance. It takes a lot of time and effort to develop a standard, and then transform it into a format that is consumable. You are paying for the time and resources that it took to turn that standard into the format that you are using and then have it delivered to you.
Many of the older standards organisations do still charge for their standards, but IMHO it acts as a barrier to wider adoption of the standards.
Many standards organisations already manage to provide their standards free. IMHO organisations like ISO and ANSI seem quite outdated by still charging.
Here is an arbitrary personal selection of important standards that can apparently be published free.
IIRC, the price of an ISO standard is directly a function of the number of pages in the standard, whatever the standard is.
Programming languages is one of the very few matter for which individuals may want to get a copy of the standard. For most matters, only companies (and sometimes an handful of them) will want a copy. And for companies, the price is indeed nominal compared to other costs (such as ensuring that their product comply effectively with the standard -- even for PL, if you are writing a compiler, 300 euros for the standard is what? 1 or 2 days for one person?, and people writing compilers, the standard libraries or books are the market for standard, most other uses aren't really pertinent).
When selling the standards, ISO is in competition with its member. ANSI, AFNOR, BSI, ... are also in the process of selling the documents. Those won't accept ISO to cut its price too much. Note that some of them sold the C and C++ for far less (ANSI had the previous version in electronic form at 30$, BSI co-published an hard copy edition; I've not found yet such a source for the latest version)
People of the committee are already contributing quite a lot of the costs (sometimes there is a fee, and then their time, they travel of their own expense, they provide sponsorship for the meetings)
Other standardization may be cheaper to get but participation can be more costly (ECMA gives freely its standards, but the fee for participation is far higher).