That's hard to say and probably varies with different pieces of equipment.  I don't mind spending more if I really feel I'm getting something for it, but like to find "bargain" pieces.  For example, my main source is an eXemplar modded Denon 2900 with full mods.  I bought it used from jpnums and then had it modded to full level.  As a result, I have $1900 into it.  For Redbook reproduction, it rivals $5K and higher.  I've heard better front ends, but they were way more than I wanted to spend. 
 
For amplifiers, I have one main one that I've sunk about $3k into.  I really like it and am in no hurry to upgrade.  Part of the problem here is that my amp is a Singlepower and now has very little resale value. 
 
For headphones, I just bought both the HF2 and HD800.  I thought that both were good buys for the money.  I also have a Grado HP2 and Headphile modded AKG K340.  I may keep all four or some may go.  It depends on what I end up listening to.