Early Boxsters have been $8 - $10K for several years. Early 996's have been $15 - $20K for several years. Of course there are some above and some below, but there's nothing to indicate values are currently in any kind of rapid decline. Anyone who predicts future values or trends is using fairy dust farted by rainbow colored unicorns. This thread alone, shows the lowly 996 is becoming a more popular car but I'll stop short of telling anyone what will happen to the prices.
If you want one, buy smart and enjoy the hobby. When I bought mine, I mentally wrote off the purchase as a loss and drive the crap out of her.
And just think - I drove her from Seattle back to Dallas and didn't have to stop and replace the motor even one time!