In my opinion, the secondary market all depends on whom gets placed in the region. We've bought tickets to the last three Sweet Sixteen trips. Two of them bought through Iowa State, and then the one in Chicago where Gonzaga was also playing where ISU lost to UVA a family member's good friend was a bigger donor for Gonzaga so we got tickets in the Gonzaga's allotment down low. The Chicago and New York markets were hot tickets because of whom else got placed there - NY had UCONN with us.
But, I have watched the secondary market for those ISU venues (as a few friends joined us and I was looking for them) and what I noticed is that the ticket prices will stay higher right now, and generally come down, unless high demand teams get placed there. In my opinion, now is not the time to buy Portland (generally speaking and painting with a broad brush here) even if you could foresee that ISU would be placed there. Waiting to see what happens is generally best for secondary market value and hope Gonzaga and some unusual draw like a lower seed St. Mary's (I guess), or someone else doesn't get placed there. I think that region will probably have some cheaper tickets though. There just aren't a lot of close teams and the pacific northwest doesn't have tremendous turnout for college hoops generally.
Anyway, my recommendation is to wait and buy through a verified third-party NCAA approved reseller until the games are announced. You'd probably be competing with Cyclone fans, but not a ton relatively are going to travel there.