I purchased a used E60 M5 In December 2012.
I looked at a few and the prices ranges from $130K to $70K.
Typically the more expensive ones have very few kms on the clock. (less than 30,000)
I acquired mine for $74K from BMW with a 12 month BMW approved used Warranty. It had 74K kms on the clock.
The challenge is that the warranty doesn't cover anything you need !
so far I have replaced :
2 front tyres
clutch, flywheel and clutch release bearing
front and rear pads and front rotors.
New NAV DVD
These items are not cheap. I have used BMW for some, but have now found a good mechanic (ex BMW) who is a LOT cheaper, and still uses genuine parts.
Having said all that I ma extremely happy with my beast. I grin like an idiot every time I start it up.
I agree with Amricho. Just bite the bullet and buy, but check the tyres, brake rotors and clutch condition first and I would do the BMW 100 point check thing too.