CompUSA has 4 Gigs of DDR2 memory for $19.95 (after $25 rebate)
I already have 4 gig in my system and two slots still available. Yes, it's 64 bit so I can support up to 128GB of memory (Vista limitation). Which, technically speaking I could buy for $384…
Back in '94, I spent $40 for a 1 meg stick of memory (and you had to buy them in pairs).