As hhEb09'1 said, very few journals that I know of are sold for $10 an issue, for a single issue or are even sold as single issues (and heck, what newsstand sells Journal of Solid State Chemistry?).

But $10 an issue for a monthly, works out to $120 per year, which is probably at the lower end of the range for annual subscriptions. So they are giving an advantage to people who buy in bulk; in other words, subscribe to the journal. Journals that are published by professional societies, such as The American Chemical Society, also usually give discounts to members of those societies, a strong incentive to become a member.

So, are the publishers and societies who issue these journals trying to subsidize their costs by "taking advantage" of the on-line, single article user - probably. But specialty, low volume journals will never be able to compete on cost to mass market magazines, because of the volumes involved and the fixed costs (the same happens with book publishing) and someone has to pay the costs.

By the way, if you don't want to shell out the $30, go visit the nearest University, find the article, and make a copy (bring change for the copying machine).

__________________
At night the stars put on a show for free (Carole King)