Well, I don't have Tivo exactly, I have the Dish PVR that came with my Dish Network, and I don't think we get charged monthly for it.
The monthly charge is for the programming service. In otherwords, Tivo works like an intelligent VCR, it gets updates and schedules of TV programming in your area, and it costs money to offer this service. DSS subscribers already get this in their dish systems, so it isn't as big a deal.
I'm pretty sure you can pay up front for a lifetime subscription, or you can pay the monthly fee.
But I'm no expert