Jeffrey is correct in saying that the first place to go should be the InDesign Server pages on Adobe.com. The 'Build a Solution' pages that Jeffrey referred to contain all the information you would need to request access to the debug version of the server for development. Once you've been granted access to the software you should also receive a PDF which details pricing for InDesign Server.
In answer to Eric's questions, the price range for a production license is typically more in the tens thousands of dollars per CPU. However, we make a version available for developers that is free of charge. To be clear, that is free to develop with, but once you have completed development, you must then buy a production license.
The EULA (End User Licensing Agreement) specifically forbids the use of the desktop version of InDesign as a server engine.
InDesign Server Marketing Manager
It's a bit technical, but i'm curious of this (also, i dont have buying power, just influence, i will have to answer to similar questions from my superior):
If i have a flash (or flex) application running in a browser that can communicate with a local desktop application to start a script on the user local machine... does it count as a "server engine" setup? (by the way, this is similar to technologies that are in developpement from adobe to let scripting application from a "flash" palette inside adobe application or from "Air" application).
If this same web application keep a list of "task" and then run it when the user click on to start all the task on his local workstation... does it count as a server engine? (does it depend if the task are added on the local station or remotely?).
The biggest reason i evaluate Indesign Server is so that a user dont have to wait in front of his computer for a long batch process to finish.
So, if a user has two computer (#1 and #2) with two separated desktop licence to his name, go to start physically the process on computer #1 and receive a automated email on computer #2 that the processing is done. Is it considered a server engine? What if the process is initiated on computer 1 from computer 2? (usage stricly limited to local network/same user?)
So much can be done just with scripting! Except for single user versus multi-user, it's not clear to me what "server engine" really cover as non-permitted scenario: remote connection (local or extranet?), connection with a database? having the user physically use the mouse/keyboard of the computer where the licence is installed?)
I have read the part of the EULA that seem to cover this... but it's not so easy to decode:
"2.3 Server Use. You may install the Permitted Number of copies of the Software on the Permitted Number of Computer file server(s) within your Internal Network only for use of the Software initiated by an individual through commands, data or instructions (e.g., scripts) from a Computer within the same Internal Network. The total number of users (not the concurrent number of users) permitted to use the Software on such Computer file server(s) may not exceed the Permitted Number. No other network use is permitted, including, but not limited to use of the Software, either directly or through commands, data or instructions, (i) from or to a Computer not part of your Internal Network, (ii) for enabling Internet or web hosted services, (iii) by any user not licensed to use the Software under a valid license from Adobe, (iv) as a component of a system, workflow or service accessible by more than the Permitted Number of users, or (v) for operations not initiated by an individual user (e.g., high-volume automated server processing of wire feed content); and"
Just tell me if for any reason, you cant adress those question on the forum.
You are absolutely correct that you can do a great deal by scripting the desktop version of InDesign. Perhaps the differentiation to make is how many users access that single license of InDesign.
In the case outlined where you have a license for InDesign and a single user, who initiates a script via a Flex or AIR application, or even two licenses for InDesign, then there doesnt seem to be problem. While there might be reasons to use InDesign Server, from a scalability perspective I dont see this as server engine use.
However, to illustrate with another case, if you provided a flex interface that initiated a script, and MANY users across the company had access, and would initiate a script running on a single machine, then that would constitute server engine usage.