Pork Trimmings can vary from 90/10% lean to 50/50% lean, to sometimes what looks like a cut-up pork butt, about 30/70 lean. A good baseline is 75/25 game/pork trimmings with very fatty pork trimmings.
If you are in deer country, the price of pork trim can go up at the beginning of deer season and stay up through the end of sausage making season, around March or so. Sometimes it costs as much as whole butt. Fatback tends to stay lower priced, and is excellent, but often fatback is sold with skin still attached. Be sure you know what you are buying and ask that the skin be removed first, or plan to remove it yourself.
NOTES: The fat must be CHILLED TO SLIGHTLY FROZEN and coarse ground by itself before added to the remainder of the recipe. The colder the better. If you are vacuum sealing, there is no reason that cannot be done ahead of time and frozen.
The meat must be also be chilled to a stiff consistency before coarse grind. Then mix the fat and meat and fine grind together.
You could also grind the meat "night before" so long as you have a good cold fridge to store it covered until mixing in the morning.