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.