Prices for CodonCode Aligner licenses managed by the Aligner License Server ("concurrent use licenses") are identical to prices for single-user licenses (detailed prices are here).
By using the Aligner License Server, you can use your license(s) at any number of computers on an alternating basis; the License Server restricts the number of concurrent users to the number of licenses purchased. The "client" computers can be any mix of Windows and Mac OS X computers, regardless of whether you install the License Server on Windows or Mac OS X.
Using the Aligner License Server is recommended for labs where licenses will be shared by different users. The Aligner License Server is a separate program that must be installed and running on a computer connected to your local network.
If CodonCode Aligner will be used only on one computer (or one computer for each license purchased), then you should consider a single-user license, rather than user the Aligner License Server. Single-user licenses are bound to a specific computer through a license key which you receive by email when you purchase a license.