I have 5kg Coastal and bhps and feed them 500gm rabbits which in fact contain far less fat than a 500gm rat. As you know, a 500gm rat is a jolly big rat, probably a spent breeder, which is packing huge amounts of fat. Feeding an animal large amounts of fat over the time period of it's adult life in basically not good. In the "wild", fat prey does not exist. And as we have established in many "discussions", neither do fat snakes! (But not exclusively because of a lack of fat prey!)
Over-feeding fat rats to inactive snakes... you do the math.
Rabbits are generally about half the price of rats, weight for weight. And if you take into account nutritional value, probably even less.
Pete, not only do I name each bunny, I give it a little farewell ceremony too! :wink: