How much is it?

Adults are $30, 4-12 is $20, children 3 and under are free.

If you are visiting for a few days and doing the tourist stuff, I highly recommend buying CityPasses.

I went at the end of may 2017, stayed one week in Seattle and bought the CityPass (79 USD), way better if you plan to visit other attractions like the Zoo, the Space Needle, and so on

Adults (13 & over): $29.95, ages 4 through 12: $19.95, and children under age 3 are free.