Yes it can be pricey, but magnesium is deleted in such small quantities that most of the time its not an issue. Ive used Mg Sulfate (Epson Salts sold at CVS, Walgreens ets.) on a number of occasions with not issues. But there are reasons behind the combo of Mg Chloride and Mg Sulfate (BRS mix).
Here is an article that explains the whole thing:
http://reefkeeping.com/issues/2006-07/rhf/index.php