Context
US$ 50 is a bill or a small monthly amount; it is appropriate to compare the bid rate and the sales before operating. You sell bonds in dollars and you receive pesos to bid rate; The other way around, you pay for the sale.
As a historical reference: a week ago (06/11) US$ 50 cost $ 79.395,00 (-0,58%); a month ago, $ 76.985,00 (+2,53%); in the latter 30 days the range was of $ 76.800,00 a $ 79.430,00 (daily sales data from ArgentinaDatos.com).
Keep in mind: It is legal and on the record. The actual price depends on the bond, the settlement period and your broker's fees.
The other way around? Passing pesos to MEP dollar.