Hi Keith,
Personally, I'm going to suggest you'll never find this acceptable.
Exchange uses a log based database - and that log grows quickly. Our database is similarly sized to yours, and even after one week, differential backups hit 5-600MB.
That said, BE has absolutely no tolerance whatsoever to a bad connection. If you get a dropout, even some lag, you'll be wondering why your backups failed.
Real backup solutions involve biting the bullet and putting a tape drive onsite. If you need a poor man's way of doing that, do a B2D onsite.
Personally, I do a full backup every day of the week. Where you need differential backups, I do full backups weekly or monthly.