The coaxial-digital cable is simply any video cable (This is what the designers of the SPDIF format had in mind: a 75 ohm video cable). So a inexpensive Radio Shack or AR Pro video cable should work fine.
Here you have some choices. But first: are you running standard component video or Progressive/HD video? Component cables are designed for a 1940's standard which maxed out at about 4 Mhz. If you are running Progressive or HD video, you really want to find a cable designed for the higher-frequency signals.
Here are some good choices:
Radio Shack has clearance prices on their "Fusion" cables. I saw a 12ft component cable for about $30. While not labeled for "HD" video, they seem to do the job.
The best bang-for-the-buck cables are the custom web sites. For about $50 they will sell you a HD rated cable similar to what is used in the broadcast industry. These cables rival the $220 Monster cables and have many happy customers here at HTF.
I'd go with the custom cable as they have the technical data to prove their cables can handle the 35 Mhz max frequency of HD signals.
In truth - the cables from target/best buy MAY actually be built with good coax that can handle HD video for 10-15 feet. But the custom cables use coax tested at 100 feet and they use some rather good RCA plugs that come close to matching the 75 ohm impedence of the cable.
People with 100" front-projectors have bought and used the Canare-based cables with great success and these systems are much more sensitive than anything I have in my home.