[Edi]

Just try with luck scale 0 and luck +3, and you'll see what happens. Luck +3 gives you a roughly even chance of getting one of the more powerful freaks (cockatrice, grotesque, ettin, another grotesque-sized foulspawn, chimera, draco-lion, basilisk) every time you cast Crossbreeding. Improved Crossbreeding gives you just more critters for less slaves, not better quality.

I've found this spell to be useful, in a recent co-op game with my friend (we stopped after everyone else was dead), I played as Arco, and found a Bloodhenge very early on. That netted me Bloodhenge Druids with 1 nature and 2 blood magic, so I got into the crossbreeding business very quickly. I could harvest slaves quickly enough to produce absolutely massive crossbred armies while buying enough hoplites was a money and resource problem.

Of course, you get a lot of absolute crap with crossbreeding, but every army needs a stack or two of fodder to soak the first impact so the better units can actually start slaughtering the enemy.

The crossbreeds turned out to be surprisingly effective if used correctly. Basically I used them as follows:
a) midgets, ground-fish (the non-amphibian idiots), werewolves and twin-headed werevolves are soaks that can and will be spent without a second thought.
b) tentacle monstrosities (red, green and lepers) are the actual useful infantry. Slow but they can be effective, as long as enemy doesn't have a lot of missile units and is up to medium or light heavy in quality.
c) spiders are good for cavalry duty, making flank attacks from sides, they can do short work of enemy archers and/or commanders if they don't get bogged down. But they eat like hogs, so supply can be a problem.
d) poison spitters are an absolutely fantastic backup for the tentacle squad, place right behind and watch the enemy die from massive poisoning.
e) big critters are to be used for maximum effectiveness in conjunction with summoned/recruited units or as bodyguards for commanders, because you just won't get enough of them to form squads of a unified type.

Of course, you need the crossbreeds in vast quantities, so for one army of useful size, you need about 90 slaves (6 castings) at luck 0, less if you have luck +3 and a powerful blood mage, so cost effectiveness is pretty limited. But using crossbreeding is not so much a pure cost-effectiveness thing as it is *fun*, or if you need to drum up a number of expendable troops in a hurry.

Edi