Hard to predict what your local rules are, but around here adding an attached structure is a headache; zoning, code, setbacks, impermeable surfaces, permits for everything, inspections, etc. If I wanted to build a detatched structure, I can go up to 800-something square feet without even getting a permit. The only inspection would be for setbacks and electrical.
I had looked into a Versatube steel structure in 24x24. The cost of the materials delivered to my house was going to be about $7000. That is JUST the structure. It comes with a big hole in the front for a garage door, and a pre-framed person door on one side. I budgeted to have someone come excavate and pour concrete. I could borrow Dad's tractor with a bucket and backhoe, but I'm not even vaguely skilled at excavation, so that's one I planned on farming out. The rest was all going to be me and some friends and pizza/beer. I figured about double the cost of the building (around $14-15k) would cover someone else doing the dig and pour, then us doing construction, purchasing the garage door, and purchasing the electrical materials (which for me would include using an existing 60A stove circuit after I convert to a gas range, adding an outside shut-off on the house, and bury conduit with cable about 20', new breaker panel in the garage, and circuits for lights, welder, and some 20A outlets). Basically everything to make it a functional shell ready to move in.
That estimate didn't include insulation, but did include an inexpensive garage door opener ($200), and a good steel person door ($300), along with a cheapo garage door ($ I forget). I had budgeted for six 8' LED shop lights which would have been sourced from a super-sale for about $20 each.
You may be able to save a bit of money on the excavation. Here in PA there are lots of farms, but they are smaller, family farms with corn, chickens, dairy cattle, etc. A lot of the farm equipment you see are combines, planters, sprayers, and plows/discs. Much of the excavation-type equipment you see is yellow, and by that I mean owned by specific excavation companies. If you live somewhere with larger farms (as the conversation suggests) you might be able to find a neighbor with a dozer or bobcat who can come over one afternoon and you give him/her $300 to dig a spot. I would have trouble doing that here.
I'm trying to remember how much Dad paid for his outbuildings. The first one was a 40x40 with a concrete floor. Morton buildings did it for him in the late 70s. He and I talked about price points when I was considering a 24x24. IIRC, that one was about $17k in the late 70s. About 15 years ago, he built a 36x48 pole shed, sided and roofed with steel, gravel floor. I think that one was around $32k. He had excavators come and make a flat pad so he could properly get drainage around it (built on a slight incline), then he dug the holes with his own 3-point auger, took delivery of the building kit, and he and I and some Amish guys built the rest and Dad and I did the electric.
I know that is nebulous and dated information, but it's all I know.