In the US you can get a 50% discount for subsequent TiVos per household (up to a max of 5, IIRC) - but for the time being we are locked into the old pricing model of 1 box = 1 sub.
The best solution, IMHO, is to get lifetime subs. That way you just feel the pain once and then it's all free
6023, Lifetime, 240gb, Sky, CC, Mode 0, 100 SPs/AWLs
6022, Lifetime, 250gb, Freeview, CC, Mode 0, 90 SPs/AWLs
601E, my 'test' machine!