I guess part of the answer is the following:
Fiscal Year 06/07: Unit sales
Turbo Coupe: 7,777 (in FY 05/06 1,517 units)
Turbo Cab: 126

GT3: 2,218 (FY 05/06 GT3+GT3 RS: 100)
GT3 RS 1,011
GT3 RSR: 37
GT3 Cup: 230

Keep in mind that when the 996 TT was launched, sales "only" reached 5,737 units.

Bottom line, since the lauch of the 2 models, the TT outsells the GT3 by 4 to 1 .... hence a greater hit on resale values.