It doesn't happen. If it did, the MBPs before the current generation would be selling for <1000 dollars. The refurbished models simply get updated; the prices hold steady. You're never going to see a <$1500 15" MBP at the Apple Store, nor a <$900 Macbook.
A friend of mine purchased a base model MacBook (I don't remember what gen) for $799 refurbed. I don't think the MacBooks have ever gone for less than that on the Apple Store.
American express is offering $100 off a $1000 Apple store purchase right now. I used it & my iphone credit to buy one of the refurb 15" Macbook Pro at $1699. I've been following the 15" prices and they have been $1699 for some time now. New model should mean lower refurb prices but Apple doesn't always do so.
since the SR update, first gen base macbooks go for $750 refurbed. The sweet spot, however, is the mid-range 1st gen macbooks: Superdrive, 1 gig ram, and 2.0 GHz. I got one of these for my parents for $850 a few months ago, now they are $800.