Rules vs possibilities

[SerenaB]

When studying on English grammar books, especially verbal tenses, I often encounter this problem: sometimes they give you rules on when to use a certain tense or when not to use it, but other times they give you examples of all the possible idiomatic or emphasided uses that are correct or allowed in only a restricted group of cases, and the difference between the two situation is not openly stated. That's what's happening to me right now. The book states that present continuous can be used (or must be used?) when you need to describe "temporary situations and and series of actions" and gives this as an example: "We aren't having any exams while the lecturers are still on strike". I've found these two phrases in an exercise to which I think the rule applies, due to the presence of expression indicating the temporaneity of the situation: "Each song only costs/ Is only costing 50 cents at the moment because they're on special offer" and "Right now, my job involves/is involving a huge amount of foreign travel".

In both cases I've chosen the first option only due to the fact that the second dind't sound right to me, and I was sure to be wrong but the keys says I'm right... But why? They seem to be pretty temporary situations to me! Thanks in advance. Please correct my English ^_^

[Safari]

You know that some verbs can never be used in continuous tenses? Maybe cost is such a verb. And involve too.

[SerenaB]

Thank you, I will check.