You can check out www.dcfever.com there is always someone selling memory cards 256MB SD goes for about $300 second hand if you want to wait. Panasonic high speed version (10mbps) is the fastest while the older SANDISK is the slowest
I stopped by the HK Computer and communication festival 2004 at Exibition center and I saw Toshiba 256MB SD card selling for only $260! I guess roughly $1 per MB is about the right price these days.