Oregon has no sales tax; Washington's is 6.5%, although certain localities have additional costs. In BC the rate is 7%.
In Oregon and Washington, anyone age 21 and older can buy pot from a licensed dispensary; these are ubiquitous and easily spotted, and they generally welcome newbies with questions. Adults can possess up to an ounce of dried marijuana. (Limits are slightly different for edibles and tinctures, but each dispensary can decide how much to sell you within those limits.) Taxes from recreational pot sales go toward schools, state police, mental health and addiction services and other public programs, so you can feel good about buying from legit dispensaries.
Note that you can't take it with you: whatever you buy in either Oregon or Washington should be consumed (or left behind) before you leave the state.