I've heard this often, and while I partially agree and understand the premise, I think that the original cost of the bike shouldn't matter.

Say that I get a POS Magna "mountain bike" from a garage sale for $15. The hubs soon turn to dust, the 6-speed SIS plastic shifters crap out, yadda yadda yadda. I score some Fulcrum MTB wheels from fleabay, a Fox fork at a swap meet, and an XTR group from Craigslist and slap all that onto the Magna frame.

Now I'd have a heavy frame with some kickass parts on it. It rolls nice, the wheels hold true, the shifters rock, etc etc. Altogether, it's a MUCH better bike than I had before, and the total cost is still a hell of a lot less than if I had bought a new Stumpjumper. Meanwhile, I keep my eyes out for a deal on a nice frameset -- and when that shows up, I've got everything ready to build it up.

Relevant to this thread, my point is, "pedals are pedals." Just because they get installed onto one old bike doesn't mean that they can't be removed later to be used on a newer, nicer bike. Might as well buy the good stuff instead of calculating its price on some percentage of the original bike price.