Oh, I get it now. Yes, under current (non-grandfathered) plans, you pay $4.95 per box beyond the first box for mirroring fees. And unless you get Total Choice Premiere, you pay another $4.95 for TiVo service (it's waived with Premiere). But that's a single monthly charge, not per TiVo. A stand-alone TiVo is something like $12/month, per TiVo.
I guess you need to find out exactly what would cause you to lose grandfather status. Since you were able to get three more boxes, apparently just adding receivers doesn't do it. It's odd that the fact that one of your receivers is a TiVo would. Is it because they're giving you a deal on the TiVo, or does the simple activation of a TiVo onto your account take you out of grandfather status? If it's the former, just go to BB or Circuit City - sometimes you can get a good deal on them for $50 or so, then activate it yourself. If possible, you need to get written confirmation of the rules of your grandfather status.