I took a closer look at the Staff of Mighty Sweeping. Its price is 28,600gp. That's 10,000gp more than a +3 weapon, making the staff likely priced out as a double weapon, with 14,000gp for a +2 and Mighty Sweeping on each end. That would place Mighty Sweeping at 6,000gp as a WSA. Or, if it really was priced for a single end, then the price for just that enchantment would be 20,000gp. I'd be inclined to believe the former is correct, given that the staff was MWed properly as a double weapon already.