I have no idea what's "worthwhile" to you, but you need to understand several things.if i did sue them for some copyright infringement, are the damages here significant enough to make it worthwhile? i'm not going to court for the 2.5k they owe me, is there a way to drag this into 5 figure range?
First, to the extent you provided any copyright protected material, you very likely also gave at least an implied license, so I don't see how there could be any copyright infringement -- at least as things stand now.
Second, if you want to sue for copyright infringement, you need to do so in federal court and you will need to register the alleged copyrights with the U.S. Copyright Office (which I assume you haven't done already).
Third, the $2,500 that you claim is not damage for any copyright infringement. Assuming you have a valid claim for that money and that the other party doesn't have any valid defense, that's a breach of contract claim. Damages for copyright infringement can include either (but not both) your actual damages (which you appear to have none) or profits earned by the infringer as a direct result of the infringement (which are incredibly difficult to prove). If you register your copyright(s) in a timely manner and any infringement continues, you might be able to recover statutory damages of $750 - $30k per act of infringement.
I'd suggest seeing if you can find a lawyer to put together a quick demand letter for a couple hundred bucks and see if that does you any good.