It's probably mostly labor cost. An 8x10 shed kit starts at under $US2k but if you pay someone to assemble it, including electrical, the price will go up very quickly.

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.