The "Cash for Clunkers" trade-in must be less than 25 years in age (your car is only 11 years old).

Generally, trade-in vehicles must get 18 or less MPG.

"Cash for Clunkers" New Passenger Car Rebate Details: The value of the credit for the purchase or lease of a new passenger car depends upon the difference between the combined fuel economy of the vehicle that is traded in and that of the new vehicle that is purchased or leased. If the new vehicle has a combined fuel economy that is at least 4, but less than 10, miles per gallon higher than the traded-in vehicle, the credit is $3,500. If the new vehicle has a combined fuel economy value that is at least 10 miles per gallon higher than the traded-in vehicle, the credit is $4,500.

It should be remembered that "Cash for Clunkers" requires you to buy (or lease) a new car. You cannot buy a used car and still get the rebate.

The official website for "Cash for Clunkers" is at http://www.cars.gov/.

Sounds to me like your dealer is just giving you a "Trade-in" amount for your 1998, to find out what your 1998 olds value should be when you trade in your car, go to he Kelley Blue Book website.
http://www.kbb.com/