For me a 180 costs:
- £60+/week fuel (250 miles upwards) - so minimum of £3000
- £800/yr Insurance (3PP, 26, N.Ireland, 4yr NCB)
- £500 (or so)/yr Maintenance (4 new tyres, oil/filter change every 6-7.5k miles, book service)
- £210/yr VED (Road tax)
That's about it i think, so the total is about £4500/yr.
From that you should be able to work out the difference in cost.