I heard that somewhere around 75% of prey weight gets added directly to a ball python's body mass. That's pretty darn efficient.

Ippo was around 150 grams when I got him at the beginning of July, and he's about 550 grams right now while having been cut down to being fed every other week for the last few months.

My two biggest females that I got in early August were originally 800 and 500 grams, and are now both over 1500 grams despite my rat shortage earlier this fall.

I should also note that the 500 gram female reached 1000 grams on a single gerbil and several mice. I don't believe in rats being any more nutritious than mice, although you need a heck of a lot of mice to add up to the same mass in rats.