Inside? Currently that would be my RAM. $180 for two gigs of top-end G.Skill back in the day. That's not counting the gig Nagasama sent me (thanks, buddy).
The computer I'm running now (desktop) is a Sempron that I built for under $100 for mobo, PSU, CPU, salvaged some old RAM and a video card and an old Compaq case, cheap USB keyboard...but I have a new 1.5TB Seagate that I leave hooked up to it. Not a gaming build, but I game on my laptop or 360.