For the record, for the model and all the extra items that come with this; I believe it is worth the money. Stardock is also selling us this package at cost which is very generous.
To clarify for LupusVir, I do not believe it comes with another activation key. You get a hard copy of the disc but not another activation key, this is why it needs your steam account linked to your digital purchase of the game to play. This is a Collector's Edition 'add-on (read: optional content)' purchase to enhance the digital copy of the game you have already purchased. The game DVD (without activation key) is for those that want a physical medium to reinstall the game from (rather than re-download everything off the wire again).
This can be found at https://www.sinsofasolarempire.com/collectorsedition

That all being said, if you set your Steam account to 'Offline Mode' on each computer and start Sins of a Solar Empire: Rebellion, you 'might' be able to play LAN.
I do know for a fact that if there are two or more unique accounts (one for each computer present on the LAN) and set them all to offline mode you can play LAN games locally without having to worry about cramming all that information out to the wire and then back again, it stays on the LAN and steam is not sucking up bandwidth.
Yarlen & crew or other forum members may be able to further explain how this works.