Because if it's good, at a cheap price, that means it's value for money. IE: The item's quality exceeds the general level of quality of other items at that price. Hence, it is a complimentary statement.
Value for money can be justified at any price. It's not a degrading connotation at all, your opinion on the term is incorrect. (EDIT: This means that it doesn't mean what you think it means)
(EDIT EDIT: People need to clarify what is being said when the person they're querying is allocating incorrect definitions to standard sayings)