I promoted to D.III for season 19. And I was also looking after a balanced defensive SF around 30k-40k. I watched the TL almost all season long.
Price I observed were around 800k to 1M,and occasionally much higher when an higher division team entered the auction in search for a sub.
Players above 30 years old have lower price, but you might loose money if you expect to sell them later. Finally I choosed a 30 years old 37k players for a bid of 560 000. He was out of Shape when I got him, so I haven't seen is full potetnial yet. But I'm rather satisfied.