I just joined a marketing company who gets paid from big companies like at&t (and other cell phone companies), dish network, GE etc and I am wondering about this companies compensation plan. So how much do those big companies pay out in advertising? Would they pay more for word of mouth?

what companies pay depends on where the ads will be seen. Super Bowl ads are notoriasly expensive becuase so very many people will see them. They pay less if the commercial is seen at 1:00 A.M. than if it seen during prime time. Many variables.
Companies also pay more when launching a new product as apposed to a tried and true product.