Contracts are usually 5 seasons, not 6, but you renegotiate generally each season or so anyways. If the show is a success in s1, you renegotiate for your s2+, if season 2 is a success... and so on. Which means when you start getting to seasons 5+ things can get really expensive as you stated. Thats a big reason why shows often fizzle out in S4-6, ratings will be down and its just not worth it to up the expense side when the revenue side is declining