There are cases (e.g. salary) where we want a mechanism for talking about an amount of money, perhaps time-qualified.
Suggestion discussed amongst the schema.org founders was to add a new type, to avoid stretching the price-related types. Therefore DatedMoneyValue or (for parallel with Price-based types), DatedMoneySpecification. Also a supporting property "currency", which is analagous to http://schema.org/priceCurrency without implying the money amount is a price.
http://sdo-venkman.appspot.com/DatedMoneySpecification is out for review.