Selling any used car is a product of supply and demand. Right now there seems to be little demand for Fieros.
I've bought three 87GT's for between $1500-$2300 in more than decent shape with 60-80K miles on the OD. If a fastback is in excellent shape then the price may be $500-$1500 higher.
Unfortunately,the market for 2M4's is very weak. I can buy them any day for $300-$700 as the collector demand for them is practically nil. The price guide may disagree with what I say but book prices and reality are sometimes very far apart.
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87GT 3.4 Turbo Best 0-60 5.2 seconds
http://turbofiero.fierojoe.com/turbo.htm