Portland is an awesome city. We think you'll have a blast. Here are some hotel/B&B options for around and under $100.
Portland is very accessible by public transportation. If you are planning on using public transportation when you visit, we think that is great, and totally doable. If you are going public transport during your trip, we recommend you stay somewhere along the Max train lines.
The Doubletree Hotel ($85 on the internet) is the best option we found, especially if you are not interested in renting a car--there is a direct train (MAX) from the airport to the Doubletree, the hotel is about a half mile from our place, many restaurants and shops, and it is considerably cheaper than the other hotels in the area. The hotel is in the Fareless Square--meaning that you can take the train for free to and from downtown Portland. The downside: it is right next to a huge mall. Getting to the Gardens by public transport is not that great. Never fear...we'd be happy to arrange some carpooling.
The Jupiter Hotel ($102) is the coolest place to stay that we know of. It is on East Burnside, surrounded by many shops (selling artist made goods and vintage) and is adjacent to the Doug Fir (Kevin's top pick for Portland Music venues), it is also not far from downtown and China Town.
B & B's
There are a ton of other options if you are more flexible with transportation. A Painted Lady Inn ($100+) is a few blocks from where we live and the Broadway neighborhood--shops and restaurants. The Bluebird Guesthouse ($50-80) looks very cute--stay in rooms named after famous Portlanders, like Beverly Cleary my fav childhood author, and Elliot Smith, one of my favorite musicians. The Bluebird is a couple miles away from us in the Southeast (think hippies, progressives).
The Hawthorne Hostel ($17+) is in one of the hippest areas of town (Haight-Ashbury-ish). Public transportation accessible by bus. The HI-Portland Hostel (about $50 for a private room for 2, but they have family and dorm rooms too) is in NW Portland. It is three blocks from the fareless square (free transportation). The NW area is trendy--lots of shopping, galleries and places to eat (this area of town can be a little pricier). Near "the Pearl", "Old Town" and "Nob Hill". Definitely a popular area and definitely accessible for public transport types.