Silvia's are way over-priced for their age i think.. My friend was after a Sr20 n/a s13... you are looking around 6.5k average for one of them (with high 190-200 kms on them)
Ca18's were a bit easier to come by, but for an auto example, you are looking around 4.5-5k.
You can find cheaper turbo models for that price!!
I have seen a few r32 n/a's scrape the 5k mark.. mainly auto though.. For something manual with highish km's you are looking for 6k or so.
Heads up - there was a s13 silvia ca18 auto on nissan silvia for 3.5k.. apparently had a cracked front bar/few dents etc.. Was in Lismore off memory