GUALEGUAYCHÚ, Argentina — On a recent cross-border shopping trip, four friends from Fray Bentos, Uruguay, visited the nearby Argentine city of Gualeguaychu, where they could afford to live lavishly and snap up eye-popping bargains. Back in Uruguay, Ferreira, 29, said that same $100 would “get your hair done and not much else.” Uruguayan businesses just across the border are finding …