Someone in the comments made a really good point. If you want to ride a bike in Portland for a few months - buy a used one off craigslist or at a shop and spend more than you want to. Then, when it's time to leave, sell it for almost as much as you paid for it. Seriously. Easily done in a market such as this.

I bought a mixte frame when we moved here. I put new tires, a basket and fenders on it and rode it to work when my commute was 1.5 miles for a few months. When we moved further out, I sold it for $75 more than I paid for it and it *almost* covered the costs of my 'upgrades'. I basically paid about $25 to ride that bike for 3 months.