You might already know this already: You could buy a UAD-1 Project Pak. That's the cheapest one. If you already own a bunch of the plugins, they will run on the "Project Pak" as well. The project pak runs around $350/new.
I don't see used cards too often, and if you do, it's usually a situation where the owner wants to get more than the price of a project pak because of the number of plugins that they have.
Put it this way, if you already have the "Studio Pak" and want to add another card, it would be foolish to buy another "Studio Pak". You could just buy the "Project Pak" instead. You would accomplish the same net result.
At least that's the way I see it.