While we were having a rough patch financially, the local grocer sold turkeys for $.59/pound. We bought 10 of them, and after trimming the breasts off for turkey nuggets for my son, I made turkey breakfast sausage with the rest. I made a basic American breakfast "loose" sausage then stuffed into hog casings. I think I still have a pound buried in my freezer (I probably made about 25lbs of breakfast sausage). One fourth of the batch had chili powder, lime juice, oregano, cumin, and garlic added to make a chorizo - which didn't last very long at all.
At a minimum, you probably want a lean to fat ratio of 75/25. Cased sausages retain more liquid than loose sausages so you can keep them a slight bit leaner. A slight bit of sweet (I used honey) won't kill you (1/2 cup of honey or a 1/2 cup of reduced apple juice) and really does a lot to the flavor.
My basic pork sausage recipe is as follows
2lbs boston butt, fine grind
4oz pork belly, loose grind
1/2 medium onion simmered in bacon fat, then cooled
1 clove of garlic cooked through with the onions
1/2 cup applesauce
celery salt, powdered ginger, sage, black pepper, red pepper flake, turmeric (not sure on the amount, I usually mix everything then fry up a piece until I get the flavor right).
Mix everything together then allow it to set in your freezer for 20 minutes. After it rests in the freezer, work quickly to run everything through the grinder together. Stuff in cases or wrap tightly (ziplocs work ok for this).
I use extra sage around the holidays, as a sage-heavy sausage stuffs a turkey very well.