How much do rats give to snakes

Say I feed a 100 gram rat to a snake roughly how much would it retain weight wise of said rat

Not scientific by any means but I've been feeding approx 60g rats to my BP and she has gained 30-40g each feeding. So to answer your question, around 50-60% or more.

I can't answer your exact question because my snakes will eat a few meals before defecating. So I can't be accurate with the exact weigh ins after each meal and poop.
But my hatchling and sub-adult snakes gain about 80-100grams a month with weekly feedings of appropriately sized rats.

Closer to 25 to 30 percent, no way they retain 50% of what goes in...

i feed my snake a 75-80g rat a week and she seems to be gaining 100g a month