I've not looked on TM for some time but I did a search for 540i today and found a couple of E39 over the 20k mark and the e60 and f10 prices seem to be dropping
8k is the cheapest E34 and E39's start at $6k the E34 tops out at $13k and the E39 at $30k with a reserve
2000 BMW 540i Motorsport | Trade Me Motors $6k down from $8.5k
1994 BMW 540i 540I | Trade Me Motors $8k
1993 BMW 540i | Trade Me Motors $10k with reserve
2000 BMW 540i E39-Fk | Trade Me Moto