Dunno the US prices, but the cheapest way is probably to buy FFXI Ultimate Abyssea Edition and register it as a second account for your wife on your PC (relatively cheap these days isn't it? Steam do it for a few of your American bucks?), setting up w/e second monthly fee ($11?).

Then buy a used copy of the Xbox360 version for next to nothing (codes already registered) and install it to your Xbox360 - you need to make sure it includes the main expansions only, ie Wings of the Goddess (all the others were included in the X360's original release), don't worry about Abyssea and mini-expansions.

You can then log in on your Xbox with either your or your wife's details, and once the ten hours of updates have run, actually play with each other!

In summary, yeah, what you said, but it can be a used copy of FFXI off eBay or something for a few bucks because you won't need the codes. Hope that helps...