I would love to help you, but its hard for me to comment unless you give me full details. What sort of tickets are you being offered?

If the tickets are some of the best in the stadium, e.g. San Martin Baja or equivalent at the Bombonera, then you are being quoted a fair price. Please note that River vs. Boca tickets are in huge demand, so there is basically zero chance you will be able to score decent tickets on your own.