Is that not always the way it goes? 10 years ago e30s (incl m3s) were cheap as they were mainly seen as old cars.
Every car goes through a phase where it's an undesirable old car only to start to gain some appreciation after a couple of years before becoming recognised as a classic.
Right now I think the E36 is probably starting to be appreciated again and is creeping up in value and the e46 is heading into the old car phase where most people wouldn't touch them. I'm still keeping the m3 though.