Just some personal experience here. My BP is currently on 4 mice every 7-10 days. A while back i decided to try and give him 1-2 larger rats mainly because it was cheaper then buying 4 mice lol. She took out the larger of the 2 rats and was not taking the second one...
Long story short that sprouted into a 1.5 month spread of not wanted anything to do with food.
From that my philosophy is this.. Most will agree that if you find something that works consistently with BP's why risk changing that? After that i went back to the mice and she is once again eating like a champion. So if it aint broke don't' fix it. From the money stand point the couple dollars you would save with the cheaper prey items is not worth you snakes health.
Perhaps rather then changing the prey item, just change the number of them giving at a specific feeding time.