Real useful reply considering NADA doesn't list this car
I'd say around $6k but the price has been really varying widely on Ebay lately.
Personally, I'd stay away from cars with the 6 lug wheels. Some of the suspension/brake parts are WAY more expensive than cars with 5 lug wheels (5 lug wheel cars use same parts as a standard DeVille). You can tell whether it is 6 lug or 5 lug by the VIN or the wheels. 6 lug VINs start with 1GEE. 6 lug wheels are all the same design so once you see one, you'll know. Lastly, all '98s are 6 lug.
Whatever you do, make sure to check for head gasket leaks. '98 and '99 Northstars are infamous for them plus you have to remove the engine from the car to replace the gasket (along with possible fooling with cylinder sleeves and timeserts).