Oh that's the one with the blue metallic paint and Ferry Porsche signature on the headrests?
Well I'm not sure that this edition is all that well-known, or that there is significant market pressure to push the price much higher than a "regular" 25th anniversary 911 of that year. Seems like, if you wanted a premium price, you'd have to sell this edition pretty hard, over the regular car, since there is a lot of confusion as to what it is.
Anyway, sounds like a really *nice* car and one that any lover of the late 80s Porsche would like.
I'm going to say somewhere between $23.5K and $29K in today's market. This presumes it is as nice as you say. My estimate is a very strong price to begin with.