well lets see a 98 Sport has either the OHV 4.0L or the SOHC 4.0L.
Now you are asking for just the installation price, and I assume you mean just a cat back exhaust system.
So if you already purchased a cat back exhaust system I personally wouldnt install it, because my exhaust shop would lose money installing customer parts, however if you find one that will they should charge you by the hour or give you a quote up front, I would say right around $100-150 would be right.
Now if you have not bought a cat back exhaust kit yet you are better off jsut having a local shop do the whole job, supply the muffler and bend the pipe, should be around $200-300 depending on which route you take.
There are alot of assumptions here, what engine do you have and what exhaust kit do you want to install? HEaders? full ehxaust? Single exhaust like stock, dual pipes? Your post needs more info
Hope that helps......