Bag or backpack?? Pretty much any backpack is going to fit under the seat (unless you are talking about those gigantor hiker backpacks). This is what I travel with and it won't break the bank. The TravelPro can fit under the seat in a pinch (and meets ULCC size requirements for underseat baggage should you ever be stuck on one).

http://www.zappos.com/p/travelpro-ma...170520001837:s
https://www.caselogic.com/en-us/us/p..._-_114_-_black