Well, I designed, and my late husband and I built, a 16' X 36' run-in, some years ago; we used 2" pipe posts, set in cement, for the supports, 2 x 6s for the rafters, , then 2 x 4s for the 'decking' to nail/screw the galv. sheets to for the roof. The walls are of the 'colored' metal panelling, and the inside is lined with wood-full 4x8 sheets of 5/8" plywood, lengthwise(so up to 4' high). This south-facing(means it faces the winter sun, and 'has its back to' the prevailing winter N to NW winds; orientation is an especially important consideration for a run-in shed!)atructure has now been in place for around 18 years!
I know this has no bearing on your question about cost(though of course, any structure would probably be cheaper if you do the labor yourself!
)-but I would like to STRONGLY suggest two things--if using metal for 'walls', LINE IT WITH STURDY WOOD-at least up to around 3.5-4', because even a mini can kick through a metal wall; I know of one who did, and bled to death before anything could be done. Also-suggest leaving some slight open space at the top of a wall/below the rafters,especially the wall directly opposite the open side, for needed ventilation, and to allow air to flow through. This should help w/ condensation, AND minimize the possibility of loosing the roof in a windstorm-at least, one where the wind blows directly INTO the open side of the shed.( I knew nothing of 'rafter ties' when I designed our shed-after all, I am an 'amateur' designer/ builder!! <grin!>), nor did I know of using roofing SCREWS instead of galv. roofing nails, when we built our shed all those years ago-about 9 years ago, during a FIERCE spring wind storm, with an uncharacteristic wind coming almost straight out of the south,a section of the roof tore off, even taking one entire rafter with it, and landed with a crash behind the shed!! There were horses in it at the time--I was hunkered down in the house, wondering if the very roof over my head was about to fly off, and heard the crash, but didn't have the nerve to even look out, so found the damage that afternoon when I went out to do the evening chores! I finally got it repaired(did it all myself-kept hoping my son could get free to come and help me, but it's a half days' drive,and he was deep in work). THIS time, I found 'rafter ties' at Home Depot, and added them to the other rafters, as well as the one I had to completely replace! Also used galv. roofing SCREWS!
Good luck with whatever you choose!
Margo