I'm another person who bought on the gate - not noticeably dearer than pre-booking, especially if you get coupons as previous posters have mentioned.
The only thing I'll say is that they'll push you towards buying the full package with the tour, IMAX film and access to the Astronaut Hall of Fame down the road, even though there are a number of different ticket options at lower prices (or at least there were when I went there last). Personally I'm a massive space buff and spent so long at the various exhibits on site and on the tour, that I didn't get time to see the film and as for the HoF, well I'd have probably needed a whole extra day if I wanted to visit that!
Just buy admission to what you think you're going to get time to see is what I'm trying to get at. Kennedy is one of the few days in the holiday that you shouldn't feel the need to rush around trying to do everything like people do in the parks (well, I do!)