Where did you get this info from? Cuz it's not correct.
All implants have the same decay. So, considering that you have to replace the Bd the least (cuz of it's high tt value), it is the most economic to use. This is because you have the least implanter decay.
So, IF you will invest, better invest in the Bd beause it wil last the longest and will allow you to se all chips.
Sorry, a bit off topic but just couldnt resist