I would be a little suspicious and very cautious, since that's a low price for that car to be sitting on a lot. I bought my '98 SVT with 123k miles, very well maintained, from a private seller who was asking $4500, and the final price was $3350, which is just below trade-in value according to NADA. As far as I've seen, '98 CSVTs in good shape with average mileage run around $4500-5000 from a dealer, which is indeed about the retail price. If there really is nothing wrong with the car though, then go for it, as it is a good deal. Chances are you could still talk the price down a couple hundred or more. As mentioned above, the most important thing to check is the transmission. If there's any grind from 2nd-3rd, that's a rather costly issue to fix.
Generally, $4000 for '98, $5000 for '99, $6000 for 2000, with average mileage. Check NADA for more specific values, but that's approximately it, within a couple hundred dollars. I wouldn't worry too much about high mileage. After doing all the research I did before buying this car, I wasn't overly concerned about buying a car with 123k miles because people reported that the Duratecs could run the 200k mark and above without any major issues. The main thing is the transmission, though - and if it doesn't grind, that's the best sign that the car hasn't been abused. Transmission wears more quickly on the cars that are.
