We replaced most of our windows last summer. (We're in Portland, too.) It wasn't cheap. We could have gone cheaper, but figured this is a big purchase that affects the value of our home, so we got the best windows we could afford, and were sure to match the style of the home. (Our previous windows were original to the house - 1913!)
It has made a huge difference in the comfort of our home. No more nasty drafts, rattling every time the wind blows, and they all open!
I think it's worth it to get several estimates, but mainly it comes down to the brand of window you buy. Some are more expensive than others (Marvin, Andersen, Pella), but I think this is one area where you get what you pay for.
Depending on the style of window you are replacing (like if they're double-hung), you can by do-it-yourself kits that replace only the window/sash and not the entire frame (like Marvin's Tilt Pac). Some look a little crappy though, IMO, and you lose some light because you are fitting a new window frame into the existing frame. Here are some links about DIY replacement.http://www.diynet.com/diy/he_windows...274801,00.htmlhttp://www.homestore.com/HomeGarden/...?poe=homestore
If you're looking for a good company locally, I recommend Classic Sash and Door, that's who we used. They carry Marvin windows. Their show room is inside the Rejuvenation store on SE Grand. Happy shopping.
And be prepared for sticker shock. Windows are not cheap. Might want to look into a home equity loan/line of credit.
One more thing. I personally think it's more frugal to buy quality the first time, than buy something cheap that will need to be replaced again.