High end stopped going low end, that killed off most of the sweet spot low end cards. Right around the launch of the GeForce1 DDR there was a switch as manufacturers started pushing the upper limits with their high end parts in terms of available technology. In days prior to that general timeframe high end parts stayed on the market until they were budget offerings, the high priced TNT eventually saw its price drop to the $50 range for a new board, same with the TNT2 and on the other side of the fence the Voodoo3s hit some low prices along with the Rage128s(pretty much everything went real low).
Now the IHVs are pushing fabrication limits and have transistor counts, and more importantly die sizes, quite a bit higher then processors. So now we have high end parts dropped when their run is done and scaled down versions being designed from day one. What has kept the market out of whack for the last couple of years has been the GeForce4 Ti4200. Because of its 'high end' core and its mid tier to low end price it was a major value and one that nV decided to keep around for a while. The competition took a while to have anything comparable and when the part was replaced, it was done with what many would say was a clearly inferior part.
Expect the trend to continue for quite some time(until after the launch of DXNext anyway) until we are nearing feature complete levels on GPUs(which are approaching). We should see a fairly linear increase on the low end starting with the next revision of budget parts, but that is going from a curve with the 5200 and 9100 parts so they are still likely to be quite inferior to today's mid tier boards. Your best bet for finding a good value over the next few years in the $100 or lower bracket is to either find a Ti4200 now, or buy a formerly high end board used when someone else upgrades.