The 300CE Cabriolet became available in 1993 and there were approx. 700 manufactured that model year. In 1994, MB changed the look on the front and rear (just slightly...but to those who love the pre-'94 W124 look, it is significant). At that time it became known as the E320 Cabriolet. It was manufactured for 2 years.
The Cabriolet is a great car! It took my husband and I nearly 1 1/2 years to find just what we were looking for (great condition, low mileage, and acceptable color) but it was well worth the wait!
If you check under any of the years '93 - '95, you will get a blue book value. The prices have come down by several thousand dollars over the past year.
The only 300 convertible model that I'm aware of from '90 would be the 300SL.
Hope this helps!