G'day danOs and welcome!
I have certainly noticed the price of second-hand 07 E92 M3's have dropped thru the floor! Dealer prices are going to be higher as they have to quote drive-away prices now, inclusive of on-road costs, stamp-duty etc. This only started in July this year. Private prices generally require on-road costs to be added so you need to take that into account.
Buying an E9x M3 is not a decision made with the brain - it is a heart thing. By this I mean you have to expect to take a hammering if you buy new however in your situation you will stand to benefit a lot buying second-hand.
One thing to consider though....a cheap second-hand M3 is most likely to have had a bit of a hammering as that is what they are designed for.
The 07 models did not have the new 09 iDrive or M-DCT option so you need to think about whether you like the technology the 09 iDrive offers (eg: on-board HDD for your music collection, enhanced map graphics etc).
The M-DCT vs 6MT debate has been beaten to death on these forums. I have the M-DCT and I love it. I love the short clicky shift lever as well as the paddles. Of course the M-DCT comes with Launch Control - which I am still to try!
Good luck with your decision!