The best prey is the one that work and can be commonly found generally rats, mice, asf ( in that order)
I do not recommend gerbils for several reasons:
1: they are more aggressive tan you average rat or mice and have larger teeth making them potentially more dangerous.
2: When BP start eating them they will imprint on them and not likely switch back, it's almost impossible to switch a BP back from Gerbils.
3: Their price and availability, they will not always be available and the cost will be outrageous when you will have to feed 2 of them a week to a full grown BP.
4: Because of all the previous reasons it could make your BP impossible to re-home should you have too one day.
The most cost efficient and convenient prey should your BP accept it is rat.
Just remember you do not need variety in their diet they can become picky enough as it is so stick to what works.
Try switching to rat an adult BP does not need anything larger than 125/150 grams weekly to thrive (female even less if it is a male)