Camping barns with power points and showers!
OK... I'll tell you what I'd expect as a bare minimum.
Four walls, a roof, a door that closes, and possibly even a window. Some kind of platform for sleeping on, rather than just the floor. Some kind of sturdy shelf for cooking on, and access to water, which doesn't have to be inside, but shouldn't be more than a few paces outside. Some kind of toilet. I'd pay £5 per night for all that, and maybe even £10 if the weather was truly lousy.
Showers, electric and gas for cooking push it to the 'luxury' end of the scale, and by the time bunks and mattresses are included, I'd start calling it a hostel, rather than a camping barn. I might go as high as £15 for all that.
One thing I really don't like are camping barns that don't cater for guys who just turn up on their own, or with a mate. I've seen more than enough barns that I'd be keen to use, but they just aren't available to me unless I pay for the entire building... up to £60 or even £100 per night. If it's going to be marketed as group accommodation, make that abundantly clear at the outset, so that someone like me can look elsewhere.