frys has good prices for a few reputable brands (ocz, corsair,,,). i don't track prices (i guess a spreadsheet would work

)
way after lowest price, i bought 24? 32? mb of edo to up a 486 from 8mb. cost about $70 (chain stores didn't stock it anymore).
about 2003, pc133 was slightly past cheapest. I paid $50 (after $30 rebate) for one 512mb dimm. 512 was largest retailed and near max for my early fsb100-133 mobos. so i use that experience as calibrator...
anyway, i'd guesstimate that ddr2 5400 (4gb, in pairs) has about bottomed out. I think at about $60 (no rebates).
eventually 8gb (in pairs) will be commonly sold upgrades. but by then, most sales might be ddr3. and 5400 will be more expensive than more common faster ram.
just now, today's midweek ad. patriot ddr2 pc8500/1066mhz (2x2gb) $100 sku 5613751 (google outpost... might give clue as to the cas, but it's likely patroir's "economy" speed) By xmas i expect ads for 8gb to barely begin. (For hi-end homebuilds, not upgrades. I'm pretty sure i haven't seen any 8gb ads yet.)
SmurfBoxMasta (d4 damager...):
watch.. and buy it while it's C H E A P
yeah. you'll get a sense when best prices (for the size you'll need to max your mobo) have flattened. then wait for the nearest big hol (black friday etc), and S T R I K E.