Where did you sink the most cash into?

CD collection.

My Silverline Boleros represent about 50% of the value of my audio system, excepting my music collection of course. 

Very nice speakers, BTW. :-)

I'm old school & was always taught & truly believe to spend 50%-60% of total budget on speakers - of course back then all you needed was speakers, a receiver, turntable & cassette deck for a system.
I still believe in spending about half your budget on speakers, then amp/s, pre, phono, cartridge, cd/sacd & cables.
It has always worked for me as no matter what source you select it will be heard from your speakers.

Most of my cash is in music.
Only 10% or less in equipment.

I started collecting records as a teenager. In looking at my collection, without a doubt, the music collection would cost more than my stereo system by far. Original Blue Notes, Six-eye Columbia's, Original Contemporary's, Riversides, Roost's, Pacific Jazz ... and then there's the classical ...