1995 Buick Lesabre

1995 Buick Lesabre 95000 miles

I am looking to purchase this type vehicle from a friend who has it for sale. She says that it need an EGR valve. I know that it has to do with the emissions but, my question what does it cost to have someone replace it and what are the pros and cons of having someone who just works on cars as opposed to a mechanic do the work?