Not sure how fast it would be, but efficient and cheap is this idea's main premise. Just get the dungeoneering bone sack thing that converts dropped bones directly to prayer experience. Then just find something that has a 100% bone drop but is weak so that you can kill it very quickly. You could play around with things that drop normal bones (only 4xp each) to things that drop big bones or even higher up bones like baby drags or something.
Depending on your stats and how many hours/day you would bot it, it could turn out to be a hefty amount of combat xp at practically no cost to you (or even profit if you get loots from the killed monsters). Just an Idea, idk what your time frame is for getting the 95 prayer or what bots you own or how much you would want to bot it, just an idea that doesn't involve having to buy and bury/guilded alter a mass amount of bones.
Hope it helped, if not at least u got a free bump, haha