This task is about reading the given passage and construct a question about the information present in the passage. Construct a question in such a way that (i) it is unambiguous, (ii) it is answerable from the passage, (iii) its answer is unique (iv) its answer is a continuous text span from the paragraph. Avoid creating questions that (i) can be answered correctly without actually understanding the paragraph and (ii) uses same words or phrases given in the passage.

Since 1934, the sale of Federal Duck Stamps has generated $670 million, and helped to purchase or lease 5,200,000 acres (8,100 sq mi; 21,000 km2) of habitat. The stamps serve as a license to hunt migratory birds, an entrance pass for all National Wildlife Refuge areas, and are also considered collectors items often purchased for aesthetic reasons outside of the hunting and birding communities. Although non-hunters buy a significant number of Duck Stamps, eighty-seven percent of their sales are contributed by hunters, which is logical, as hunters are required to purchase them. Distribution of funds is managed by the Migratory Bird Conservation Commission (MBCC).
How much money has the sale of Federal Duck Stamps generated since 1934?