There are a couple of options, the D or 4 train to 161st street, which is right by the stadium. There are also MTA trains on the Hudson, Harlem, and New Haven lines that drop you at 153rd street E, which leaves you about a $10 minute walk to Yankee Stadium. If you go the subway route, make sure you buy your round trip ticket ahead of time. The last thing you want to do is be standing in a line after the game, needing to buy your return ticket.
As far as cost, your round trip subway ticket to the Yankees game is going to be about $6 per person. Here are some links to more information:
MTA Yankee Stadium Summary
MTA Sports Page
Official Yankee Page