Newegg has the Transcend 4 GB SDHC Class 6 Flash Memory Card TS4GSDHC6 for $6.99 with free shipping. There are other companies with lower prices before shipping, but this is the lowest shipped price around on this 4GB card and you're saving about 50% off the next best price after shipping.
Transcend Secure Digital High-Capacity (SDHC) Class 6 Card is fully compatible with SDA 2.0 specification. It is suitable for SDHC compliant devices, SLC flash chip with High Speed transfer rate. Perfect for high-end digital devices.