Traveling in Time, not yet or are we?

Carl Sagan in an interview with Nova on Time Travel discusses time travel and why we can or cannot time travel, yet.  Perhaps future time travelers can only go back to the time when the time travel machine is invented but no further.  It doesn’t mean that it can’t happen, just not yet. He also indicates that perhaps the cost is too great to go too far back into the past, but I don’t think that argument holds water. If we have people who can buy a trip to the space station, time travel is not cost prohibitive. Nova also asks Sagan about the Grandfather Paradox, where if you can go back in time, and kill your grandfather before he sires one of your parents, do you exist?  If your grandfather is dead, how did you get into the future to go back and kill him?  Sagan does not believe this is possible, hence the paradox, but could you not meet your ancestors and affect how they live their lives for better or worse which in turn will affect how the offspring turn out, in essence, you. We do travel in time, every year, every month, every day, every minute and every second we are moving into the future.  I find the concept of time travel to be intriguing.