childcare for twins..

Al
Those mommas that have twins, how much do you pay/would you be willing to pay for childcare for your 8ish week old twins to go to a babysitters home? 
I'm currently 25 weeks pregnant with one baby girl and going to stay home with her. Our good friends just found out their pregnant and with twins! I am going to be their babysitter but I have no idea what to charge. I don't want to rip them off but I want it to be worth my while especially with twins..