My thought on this is that there is no free lunch. Someone has to pay for this data retention, for the spinning rust, the tape archives and the masses of servers sitting up top. Who is supposed to pay for that? The ISPs? Are you not attempting to transfer the true cost of the surveillance tools you believe are necessary to perform your job?
What about the security of the data? It is very expensive to ensure the data remains secure both during transfer and at rest. Who is going to pay for continual audits and penetration testing? Who is liable when a user's privacy is violated?
No. If it would blow your budget in a week and not turn up the quantity of useful leads where you can justify it against your own budget then that probably tells you something about its value. Just pushing the cost to someone else doesn't make it any cheaper. It just makes our internet bills higher rather than our tax rates. Forgive my lack of excitement over that "saving".