If you're actually using it 8 hours a day then it's 30 watts for the mba, and 150 for the pc + 30 for the display. So the difference is 1.2kwh per day. For 200 days of work in europe, that's 200
1.2.4 = 96 per year.
In the US that's 235 working days per year at around $0.20 (13 - 25ct), so about $50 per year.