this is frequently discussed on the W Europe branch.
the consensus on this branch is that you need about EUR 45-50 per person per day for W Europe as an absolute minimum. This includes a dorm bed or cheap shared double room, 3 self-catering or fast-food meals, local transport and one activity per day. It excludes long haul travel, drinking sessions, full sit down meals, and lots of expensive attractions.
However, i guess most people would spend more than that per day.Depends on what you do.

