I usually see a few hotels, including the THE PORTLAND HOTEL - MANCHESTER (becoming an ibis Styles) that is really well located and when available, costs £25. I almost booked it just to try it out (and to see if they upgraded me to a suite as a platinum member), but as I didn't need a hotel and they don't provide breakfast I decided not to.

For next weekend you can get the Mercure Manchester Piccadilly Hotel for £40 per night for example (friday and saturday night). I don't remember seeing the Ibis (princess street) on happy mondays though.