I personally think you should pass on this one. At $11,500, you're within striking distance of getting at least a West Hurley that was originally made with the full-auto receiver dimensions. This will be very important down the road, regarding parts interchangeability, etc. The days of rapid price escalations in machine guns are over (mainly due to the lousy economy), so you shouldn't feel pressure to jump on the bandwagon right now.
Bear in mind also that a Thompson is something that has sentimental and historical importance that outweighs its practical usefulness as a weapon (it's too heavy and awkward in use). If you're looking for a practical weapon, a registered-receiver M16 would be a better bet at about the same price. On the other hand, if you want a wall hanger, you can put together a very nice display (non-firing) Thompson, using an IMA parts kit and Philadelphia Ordnance receiver, for about $2,000 -- and this would have no legal red tape and be more "authentic," in its way, than a West Hurley Thompson.
But what can I say? If you really want it, go for it.