1973 was a great year for a ragtop Vette in driveable condition for $8k!

Yes, engine performance started to suffer after 1971. Most of that is detuning for the eventual arrival of the catalytic converter by reducing compression ratios and adding smog controls, such as the air pump to the engine. Catalytic converters weren't added until 1975 (probably earlier in California). Pre-1975 models can be tweaked for older L82 horsepower levels without too much effort.

Anyway, unless it is an LT-1 or some other very rare model, the real value is in what shape it is in. If it is a completely original L-82 with low miles, you value will obviously be higher.