Processor has an interesting analysis here on the comparative costs between iSCSI and Fibre Channel, based largely on a report by Forrester's Andrew Reichman. The upside is that iSCSI is most certainly cheaper -- nine times cheaper in some scenarios -- but may not be robust enough for some applications.
Reichman compared the overall cost of network fabric technology -- everything from NICs and switches to actual cabling -- although not actual servers or disk arrays, which some say can be affected by the type of SAN protocol you're using. Among some of the tidbits is that even though iSCSI lets you get away with the built-in NIC on most motherboards, you're giving up about 80 percent of CPU efficiency to extensive overhead requirements. iSCSI is probably the choice for dual-path networks considering most OSs offer native support for iSCSI multipathing while Fibre Channel needs expensive proprietary solutions.
But even though iSCSI wins on price, it isn't necessarily the cheapest option out there. DAS arrays using SAS drives, and even many NAS systems, can be installed for less while still providing adequate storage connectivity for many infrastructures.