Apple typically only changes RAM prices at revisions. If the market fluctuates they don't change the BTO option prices until the next time the update that machine. When the 24" iMac added 750GB drives, I think the 500 came down a little, but I could be wrong.
Edit. Funny you got the 2.33 but not the RAM, HDD, or VRAM upgrades to your 24".
Most people suggest VRAM, HDD, RAM, CPU upgrades in that order, maybe RAM, then HDD. CPU is usually last on the list, b/c cost diff of 2.16 -> 2.33 doesn't seem worth it.
Of course I almost got a 24 and would have gotten a 2.33, but also 2GB, 500GB, 7600GT.