Hello, I'm Jason and will helping you with your Camaro value question. So there are a few ways to describe cars, depending on production date, however, there is no official "semi antique" classification. There is generally antique and classic. A 15 year old car wouldn't fall into either of these just yet. Classic is 20, antique is generally 25, but some information sources vary on what is considered antique.
So your Camaro is relatively scarce, with total Z28 convertible production being about 2500 out of 77,000 Camaros built for the '98 model year.
However, unless this car is of concours quality, the value isn't going to be overly strong.
If it's a perfect car in all respects, it could sell for $16,000
If it's driven and has very few flaws, and needs no mechanical repair, then $13,500 The average 1998 Camaro Z28 Convertible will fetch about $10000, with examples needing repairs significantly less than that