You need a valid license for each computer you intend to install Mac OS X on. Spending $129 on one copy of Mac OS X and installing it on multiple machines is illegal. The true cost would be $129 multiplied by the number of computers you intend to install it on.
Also, you cannot use a USB cable to "network" the computers in a fashion that would allow you to clone one installation to another. The workaround is to use a FireWire cable between the computers, start the target computer up in "FireWire Target Disk Mode" by holding down 't' as the computer boots, then use software such as Carbon Copy Cloner to "clone" the Mac OS X installation on the master machine to the target machine.
Mac OS X isn't like Mac OS 9 in the sense that in Mac OS 9, you could simply copy the entire hard drive to another hard drive and retain all functionality and bootability. You cannot simply copy the files from one Mac OS X installation to another due to UNIX-style permissions -- the resulting copy would not be bootable. Carbon Copy Cloner takes care of these issues and correctly sets permission bits (among other things) to produce a fully bootable "clone" of your installation.