Edit: I mean I've seen boxsets in BestBuy for as little as $25 and they're shows I watch.
If I had all the series of Simpsons, Futurama, Mythbusters, Family Guy and American Dad I believe from then on I believe I would rather use the monthly subscription fee (about $150 here for all the channels we want; TMN is great to have as they're free of charge for a monthly fee, I get all the recent Stargate and Dexter episodes and most of the big movies not too long after they're out for rental) for buying more boxsets and movie rentals.
How much does HBO charge a month?