It is completely dependent but most Porsche's hold their value and sell for more than your average car.
An old 356 will go in the $25k + range, a Boxster depending on milage will go from between $11k-60k, a new 911 will go for about $40k-$80k, and older 911's sell in the $25k-$40k range. Then there are the 944, and 928 that can be found between $1k-25k.