For us it is absolutely worth it. We did a party in 2010 and loved it so much we booked two last year and both were, again, great evenings.
I think you have to ask yourself first:
Will you stay until the end of the evening (or close enough to it)?
Are you willing to focus on the party events, rather than be tempted to run for rides all night? (There is a common attempt to justify the cost of the event based on 'short lines' which although is fact for the most part, with parks being relatively quiet during the Fall season, it is not a heavy enough swaying factor on its own).
Would the atmosphere in the park on the party night (which is completely different to any other time) be seen by you as an added experience that would go towards backing the value of the extra expense on tickets?
Do you want to meet characters and have your photos taken/get autographs from characters who otherwise do not appear for meet and greets?
If you answer no to the first one, you may need to consider very carefully. But if you answer yes to that and any one of the other questions, I believe it is worth considering going to the party.
If you do decide to go, my advice to you is to keep up to speed with all the threads, blogs and posts on this year's party (which will be under way by the time you are ready to leave so you will have the benefit of some live and current reports). Do your homework. Have a wish list of things to do. Find out where everything is and have a plan. Take into account the usual 'rule' where parades make it difficult to cross Main Street for a period before, during and after they are running and plan accordingly if you intend navigating for example from Tomorrowland to Frontierland.
There are dance parties which are great fun, the Villain's Mix and Mingle is brilliant and the general atmosphere is absolutely intoxicating (and the party mood very contagious!)
If you are purchasing or have pre-purchased Photopass, make sure you know where to go for the special effect shots as these are great and you will only ever get them at the party. (I love photos so this is always an easy sell for me

)
I can't tell you that it's worth the cost for you as there are only two of us, so I can see why you are weighing this up so carefully, but if you can afford it and you look at what you want out of it, I do think its something worth doing at least once.