Electrical + AC is a firm line where that goes from shed to detached office.
I know someone who builds horse shelters that are larger than that, but a fraction of the cost, delivered. I've actually been thinking that if I wanted an outside office, I'd ask him to custom build one with the fourth wall closed (horse shelters are 3-sided) and I'd insulate and sheetrock it myself. IIRC he did our 10x30 one for about $2k, including delivery.
Around here, the cutoff for needing a permit for a "permanent structure" is 10'x12' which is a pretty good sized outside office.
Labor is expensive. You could probably close to double the size for an additional $10,000, you’d need another yard and a half of concrete for the foundation (6” depth*80 sqft = 40 cubic feet) and more material for the building, but not twice as much material, or labor. Electrical service could stay the same.l and the mini-split wouldn’t have to be much bigger.