My bill raised by £9.50 per month: £6.50 for the M+ TV package (a minimum for 500GB TiVo) and £3 for the TiVo service itself. I was hesitant at first but I'll be damned if I can't watch broadcast TV without it!
As a side note I filled up the hard-drive even just on M+ with 500GB TiVo to about 70% within a few weeks of ownership, hours of shows to enjoy, demonstrating that you don't necessarily need the L or XL packages to get your use out of the box. To be fair though, it was Rugby World Cup and I had everything recording from that in HD.