Intake Manifold Gasket?

I just took my 2002 Ford Explorer Sport Trac to Firestone. They said my intake manifold gasket is leaking. They suggest that I not only get a new gasket, but another intake manifold as well, bc accoding to them, its better to get it all in one fail swoop bc the manifold itself is likely to fail. It sounds like a crock to me to get me to spend $920 which is what I was quoted. Am I being to harsh? Also just to seal it with a new gasket should run me what? I know its an approximation, but a ballpark figure would be good to know.