What is the different between a 5series and m5?
 

just take the infamous e39 m5 and compare with the non-m e39 5 series.

engine: Non-m INLINE-6 2.5L, 3.0L and a V8 4.4L. M5 - modified V8, better engine cooling (non-m water pump $140, m5 water pump $400), individual runner intake...the list goes on. basically, the entire engine is different.
drivntrain - reinforced transmission, driveshaft, drive-axle AND a limited slip differential.
brakes - ALOT bigger brake caliper and rotor.
suspension - retuned and upgraded
subframes - retuned and upgraded
tires - ALOT wilder
interior - upgraded and bmw always offer special edition items, both on m cars and non-m cars, these special edition item are rare and worth more money. (for example, the m5 that my friend is parting out right now has the leather stitching dashboard set which is worth double than a normal m5 dashboard set)
exterior - the m body, m front bumper, m side trims, m rear bumper, m exhaust which is quad-tips