I too buy less mayo, but that's a matter of trying not to waste things I'm never going to eat. I do see a drastic difference with food and cable channels going unused though.
Regarding the other aspect, you still end up paying less. Sure you get less, but you get everything you want. I don't pay my cable bill, so I have no idea what it costs, but lets settle at an arbitrary $50 per month. At $600 a year, you'd get the shows you want (the three you listed) and a ton of stuff you're never going to watch. Now, what if Amazon or whoever placed those shows at $201 per season. You can get them for $603 for the year, and you get everything you want, and nothing you don't but it costs not even a penny a day more. But would you? No, you buy them for about $30-40 each season. That's about $120 a year (at most). Still a tremendous savings under $600. But what if Amazon did jack the prices? Would you pay $80 each a season? Would you pay $100? $120? $150? $200 to exactly equal a full years worth of cable? I can't imagine anyone would bite. I can imagine that Amazon negotiated a price range which allows A) the studio to be happy, B) Amazon to grab a profit, and C) the consumer to actually pay for just three shows.