You pretty much answered your own question in your first post. It's about condition, mileage, turbo vs non turbo, auto vs 6 speed. There's not really anything secretive about it. There's no "Type R" model or anything like that. Most of the high dollar Supras you're probably seeing are the 97-98's which are the last years made here in the US.
A mint condition, turbo, low mileage 98 will fetch WAY more than a built 93-94 just about any day.
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09 Civic Si