I saw field shelters made by Ellipse Fabrications at Border Union Show on Friday, and was very impressed by them. My husband built ours (he's an engineer as well as a farmer, so does know what he's doing) and they are great, but comparing the cost of the materials we used plus his time, these came our favourably!
The link is to a facebook page, but you should be able to see everything even if you don't use facebook yourself.
The shelter in the photos is £1600 + VAT.
The company is nothing to do with me, and I'd never heard of them until Friday, was just mightily impressed by their product. The chap said it takes apporx 3 hours to build and 1 to dismantle.
https://www.facebook.com/ellipsefabrications/?fref=ts