Here's a thought. What if your ship can only do one sort of test per specimen, before it dies and breaks down? Then you do different tests on each one for a little data each time. As for the Evil Ones, you lie to the Spathi about still having them alive. ZEX's Beauty? Unique, so it gives a lot without any tests. But as for the price or why they want them, I don't know.
Perhaps the different amounts of data received depends on the creature's resistance to death, as opposed to stunning.