Nissan Altima: 97 altima exhaust problems

Hi. I've got a 1997 Altima that has suddenly become a problem. About three
weeks ago, the service engine light came on, so I took it to the garage to
have it checked. They said the code was for the mass air flow sensor. They
also said there was a problem with the exhaust system. The charge for
replacing the maf sensor was $600 and they wanted another $500 to fix the
exhaust. I asked if having the maf sensor replaced and waiting a month or
so on the exhaust system would hurt anything. They said no, so I had the
maf sensor replaced but nothing was done about the exhaust. Now, three
weeks later the service engine light is back on and they are saying I need
a new egr valve and egr sensor. Are all of these things related? How much
should I expect the egr valve and sensor replacements and work to cost?
Would fixing the exhaust solve all these problems? I hope someone can help
me out. I'm waiting until Monday to tell them what to do. Thanks in
advance.