Around 600 yen. A bento at a convenience store costs about 450-600 yen. A bottle of Coke is 147 yen. If you go to a 100 yen store, you can buy 5 items for 525 yen, but generally they are Chinese products. If you buy small accessories, such as mobile phone accessories, etc. at the airport duty-free shop, the cheapest ones cost 300-600 yen. To sum up, 50 yuan can buy at most 1 to 2 small commodities that may be of interest to you.