One load my nephew and I got was store paneling, which filled our truck, and was very heavy. It went to Houston during the summer. We were told there would be someone there to unload us... there wasn't. It took us all day, and every bit of strength we had. I believe we got 450 bucks for the hand unload.
The OP isn't very far off from what I would offer, being that what he would have to unload would be a complete unknown until he gets to the shipper. Xiggi has the right idea; but if we're talking a 24'er here, I'd go with 150 for the first hour - min, plus 75/hr afterwards, or 450-500 flat. Of course, this would be different if it ended up unload-able w a pallet jack, or only took up 10', for instance. But sight unseen, along with the broker not having a clue what the deal is, you definitely charge for the undisclosed.