If more people go to Yankee games because Jeter is playing, I would guess its negligible. It's purely hypothetical, but I'll give you 2 scenarios:
A) Jeter is signed, team wins 90 games.
B) Jeter is not signed, player of equal quality is signed for 2/3 the price, money saved goes towards another player. Team wins 92 games.
I would argue that the team in scenario B would be more popular, and draw more fans. And if the cutoff to make the playoffs was 91 wins? The team would be MUCH more popular in scenario B.
Again, were assuming the Yankees have a max budget, which probably isn't the case. It doesnt really apply. But just look at attendance figures - the good teams draw well. The great teams who make the playoffs draw even better. The 1981 Yankees, a WS team, was 1st in the AL in attendance. In 1991, they were 11th of 15. In 2001, they were 2nd.