EDIT: Apologies, just noticed your budget. I’d still recommend the Deuter though.
How big is your, err, machine? And what else do you want to carry?
I’ve just had this conundrum thanks to starting to cycle to work, and went for a Deuter Trans Alpine 30.
http://www.deutergb.co.uk/bike/trans-alpine-30/
Accommodates my 12incher Thinkpad X230, iPad, all the cables etc, shoes, a full set of dressy ‘business casual’ clothes, wash gear, and lunch. I reckon it’d cope with a 15in laptop fine as long as it wasn’t a desktop replacement. Has loads of pockets in sensible places, good rain cover, reflective, and generally really well thought out design.
Currently on sale in more subtle black from CRC:
http://www.chainreactioncycles.com/deuter-trans-alpine-30-backpack/rp-prod88941