If you want to get unlocked versions of both the regular and dual SIM versions of the Lumia 640, or the larger Lumia 640 XL, then Expansys USA might be something to check out. The online retailer has cut the price of those Windows Phone devices for this weekend only.
At Expansys USA, the 5-inch Lumia 640 in its single sim version is currently priced at $179.99, a $30 reduction from its normal price. The dual SIM version of the same phone is priced at $189.99, which again is a $30 price cut. The 5.7-inch Lumia 640 XL is on sale as well for $249.99, a $40 reduction from its normal price. All three phones come in black, white, orange and blue colors.
Keep in mind these smartphones are the international versions and thus may not work on all wireless carriers.