If you want to save some cash, go on 2nd Tuesday of each month for $4.  Hoping the crowds will be small on this date because everyone will have gone the day before.  Just east of Portland on Hwy 26 in Washington Park.

http://www.oregonzoo.org/visit/hours-admission-special-offers

Adults (12–64): $11.50
Seniors (65 and up): $10
Children (3–11): $8.50
2 and under: Free