I had a lot of trouble finding an affordable bag that would fit my laptop, an Inspiron 5100. 5100's are kind of thick and the 15" screen doesn't help either.
I also didn't want to give up my current pack so I just bought a laptop sleeve from
www.ebags.com
Sleeves cost about $20 and I've had no problems using my daily use messenger bag or my standard pack when I bike somewhere.