In contrast to certain other open-source databases, PostgreSQL leans toward protecting data from surprises and erroneous input, i.e. rejecting a date of 2013-02-31 instead of arbitrarily assigning a date of 2013-03-03. Similar "throw error" instead of "take a guess" philosophy applies to numeric and string operations as well. It's an approach I appreciate.
But it appears that the philosophy does not extend to the "money" type. Although there are certain checks including no alpha, '$' and '-', if present, must be in the first two characters of the string and commas can't be at the end. Otherwise the casting is fairly liberal. Commas, for instance, can appear nearly anywhere including after the decimal point: