Which is correct and why?
Each song only costs/is only costing 50 cents at the moment because they're on special offer.

Right now, my job involves/is involving a huge amount of foreign travel.

Question

Which is correct and why?
Each song only costs/is only costing 50 cents at the moment because they're on special offer.

Right now, my job involves/is involving a huge amount of foreign travel.

Answer

1) costs
2) involves

The verbs "involve" and "cost" are non-continuous.  It means that they are never or rarely used with continuous forms.
http://esl.about.com/od/grammarintermediate/a/noncontinuous.htm

Answer

Each song only costs...
My job involves a...

Answer

не зависимо от времени это S -вообщем. продолжительность во времени и действия ING сейчас или в прошлом