I've recently started using my zed for work trips, and you'll do a lot better than 20mpg!
On a motorway run i get 33mpg without trying really... just think ahead and try not to touch your brakes.
On 2 separate runs to Norwich and back i've managed 36 mpg, but that is all single lane A roads at 60mph for 360 miles.
Use the following website to work out your fuel cost:
http://www.fuel-economy.co.uk/calc.shtml
Its sad, but i have a spreadsheet with all the locations that im likely to have have to drive to for work, and how much it will cost if i take my zed or my other car, so before making the trip i can check which one will work out best.